Spatial Receptive Fields Research Articles

Layer dependent visual receptive field properties in ferret superior colliculus

Event Abstract Back to Event Layer dependent visual receptive field properties in ferret superior colliculus Iain Stitt1*, Edgar Galindo-Leon1, Florian Pieper1, Gerhard Engler1 and Andreas Engel1 1 Institute for Neurophysiology and Pathophysiology, Universitätsklinikum Hamburg-Eppendorf, Germany The mammalian superior colliculus (SC) is a highly conserved midbrain structure that responds to novel external events and initiates orienting movements. Interestingly, all layers of the SC, regardless of modality, are organised into juxtaposed, overlaid maps of space. Recent anatomical studies have provided evidence for a direct superficial-to-deep projection in the SC. However, it remains unclear how much information is conveyed by this connectivity. Here, we recorded single and multi-unit activity, along with local field potentials from both superficial and deep layers of the SC with a dual-shank silicon multichannel probe. Spatial receptive fields of the neurons were determined using small (~3°) drifting gratings presented at 20x20 different locations on a rear projection screen. The location of spatial receptive fields recorded within individual SC penetrations were very similar, as predicted by anatomical connectivity. To assess directional/orientation tuning characteristics across SC layers, full field drifting gratings were presented with 8 directions spaced equally over 360 degrees. Of visually responsive units, 22% were classified as orienatation selective, 7% of were classified as direction selective, and 5% units displayed characteristics that were intermediate to both classifications. Correlation in feature selectivity between recording sites was greatest at neighbouring electrodes, and decreased proportionally with distance between recording sites. Previous studies have focused on the visual receptive field properties of neurons in the superficial, retinorecipient layers of the SC. In this study, we record from the entire laminar structure of the SC to assess the physiological relevance of the anatomically defined superficial-to-deep projection. Keywords: Perception, superior colliculus Conference: XI International Conference on Cognitive Neuroscience (ICON XI), Palma, Mallorca, Spain, 25 Sep - 29 Sep, 2011. Presentation Type: Poster Presentation Topic: Poster Sessions: Neurophysiology of Sensation and Perception Citation: Stitt I, Galindo-Leon E, Pieper F, Engler G and Engel A (2011). Layer dependent visual receptive field properties in ferret superior colliculus. Conference Abstract: XI International Conference on Cognitive Neuroscience (ICON XI). doi: 10.3389/conf.fnhum.2011.207.00361 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 23 Nov 2011; Published Online: 28 Nov 2011. * Correspondence: Dr. Iain Stitt, Institute for Neurophysiology and Pathophysiology, Universitätsklinikum Hamburg-Eppendorf, Hamburg, Germany, i.stitt@uke.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Iain Stitt Edgar Galindo-Leon Florian Pieper Gerhard Engler Andreas Engel Google Iain Stitt Edgar Galindo-Leon Florian Pieper Gerhard Engler Andreas Engel Google Scholar Iain Stitt Edgar Galindo-Leon Florian Pieper Gerhard Engler Andreas Engel PubMed Iain Stitt Edgar Galindo-Leon Florian Pieper Gerhard Engler Andreas Engel Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

A model of retinal ganglion cell processing under natural viewing conditions.

Event Abstract Back to Event A model of retinal ganglion cell processing under natural viewing conditions. Garrett Greene1, 2*, Thomas Wachtler1, 2 and Tim Gollisch1, 3 1 BCCN München, Germany 2 LMU München, Germany 3 Georg-August-Universität Göttingen, Germany The retina is an important first stage of visual processing and is intensively studied. Little is known, however, about the retinal responses under atural viewing conditions. In particular, it is interesting to consider whether standard receptive field (RF) models can capture important features of the retinal response to complex non-stationary stimuli. We present a conceptually simple model of the peripheral visual system and investigate its behaviour under simulated eye movements. Specifically, we identify possible retinal mechanisms for rapid post-saccadic signalling as observed in several experimental studies. We model the retina as a simple photoreceptor grid which feeds forward to a retinal ganglion cell (RGC) layer consisting of numerous distinct RGC populations. We model four such populations: linear midget cells, both On-type and Off-type, and non-linear parasol cells, On-type and Off-type. Spatial receptive fields are given by a difference-of-Gaussians model. Temporal integration profiles for each population are described by the difference of two bi-phasic temporal filters which are combined with the spatial receptive field to form a rank-2 spatio-temporal filter. For midget RGCs, this filter is linearly convolved with the stimulus and rectified to generate a membrane potential. Neuronal spiking is simulated by a threshold function with non-linear feedback. Parasol cells implement a non-linear spatial integration, in which the receptive field is divided into sub-fields. Contributions from each sub-field are temporally integrated and rectified, before being summed to provide an estimate of the parasol cell membrane potential. The standard centre-surround RF can be recovered by combining sub-field RFs. The model reproduces several salient features in the reported behaviour of mammalian retina, including spatial and temporal modulation transfer functions. In addition, numerous studies have reported rapid post-saccadic bursting in retinal sub-populations. This behaviour is reproduced in our model, which displays strong transient responses to simulated saccades on a 10-ms timescale. These responses are observed primarily within our Off-type parasol population, and are less pronounced in On-type parasol cells. Midget RGCs do not display post-saccadic bursts. These differences in populations arise as a result of differing spatial and temporal integration profiles and the parasol integration non-linearity. Such dynamics are also implicated in the generation of latency codes for rapid post-saccadic processing. Our model thus captures prominent features relevant for understanding retinal processing during natural vision. Keywords: Eye Movements, retinal ganglion cell, sensory processing, Vision Conference: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011, Freiburg, Germany, 4 Oct - 6 Oct, 2011. Presentation Type: Poster Topic: sensory processing (please use "sensory processing" as keyword) Citation: Greene G, Wachtler T and Gollisch T (2011). A model of retinal ganglion cell processing under natural viewing conditions.. Front. Comput. Neurosci. Conference Abstract: BC11 : Computational Neuroscience & Neurotechnology Bernstein Conference & Neurex Annual Meeting 2011. doi: 10.3389/conf.fncom.2011.53.00224 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 11 Aug 2011; Published Online: 04 Oct 2011. * Correspondence: Dr. Garrett Greene, BCCN München, München, Germany, greene@bio.lmu.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Garrett Greene Thomas Wachtler Tim Gollisch Google Garrett Greene Thomas Wachtler Tim Gollisch Google Scholar Garrett Greene Thomas Wachtler Tim Gollisch PubMed Garrett Greene Thomas Wachtler Tim Gollisch Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Maintenance of enriched environment-induced changes of auditory spatial sensitivity and expression of GABA A, NMDA, and AMPA receptor subunits in rat auditory cortex

Enriched environment (EE) has an important role in the development and plasticity of the brain. In this study, we investigated the maintenance of early EE exposure-induced changes of spatial sensitivity, and the possible underlying mechanisms of this maintenance. We found that, compared with the age-matched control, the spatial sensitivity of A1 neurons was still enhanced after EE rats had been returned to the normal condition for 2 months. The enhancement was expressed by a sharper frequency tuning curve, smaller spatial receptive field, and a more selective directional curve of the early EE-exposed rats. Simultaneously, we detected significant increases in GABA A receptor α1, β3 subunits; NMDA receptor NR2A, NR2B subunits; AMPA receptor GluR2 subunit protein expression; and in the ratios of GABA Aα1/GABA Aα3 and NR2A/NR2B. In particular, the expression ratio change of the GABA Aα1/GABA Aα3 was significant greater than that of NR2A/NR2B in early EE-exposed rats. These observations indicate that the persistent higher expression levels of the GABAergic and glutamatergic receptors expression induced by early EE exposure, especially enhancement of GABAergic inhibition in the auditory cortex, might be responsible for the maintenance of improved effects in auditory spatial sensitivity after the rats had been returned to the normal condition.

Soil erosion hazard maps for corn stover management using National Resources Inventory data and the Water Erosion Prediction Project

A topographic map of auditory space is a feature found in the superior colliculus (SC) of many species, including CBA/CaJ mice. In this genetic background, high-frequency monaural spectral cues and interaural level differences are used to compute spatial receptive fields (RFs) that form a topographic map along the azimuth. Unfortunately, C57BL/6 mice, a strain widely used for transgenic manipulation, display age-related hearing loss (AHL) due to an inbred mutation in the Cadherin 23 gene (<i>Cdh23)</i> that affects hair cell mechanotransduction. To overcome this problem, researchers have used young C57BL/6 mice in their studies, as they have been shown to have normal hearing thresholds. However, important details of the auditory response characteristics of the SC such as spectral responses and spatial localization, have not been characterized in young C57BL/6 mice. Here we show that 2-4-month C57BL/6 mice lack neurons with frontal auditory RFs and therefore lack a topographic representation of auditory space in the SC. Analysis of the spectrotemporal receptive fields (STRFs) of the SC auditory neurons shows that C57BL/6 mouse SC neurons lack the ability to detect the high-frequency (&gt;40kHz) spectral cues that are needed to compute frontal RFs. We also show that crossing C57BL/6 mice with CBA/CaJ mice or introducing one copy of the wild-type <i>Cdh23</i> to C57BL/6 mice rescues the high-frequency hearing deficit and improves the topographic map of auditory space. Taken together, these results demonstrate the importance of high-frequency hearing in computing a topographic map of auditory space. <b>Significance Statement</b> Despite the strain’s age-dependent hearing loss, C57BL/6 mice are widely used in auditory studies because of the development of transgenic reporter and Cre lines in this genetic background. Here we examined the topographic map of auditory space and spectrotemporal properties of neurons in the SC of C57BL/6 mice. We found an early-onset high-frequency hearing deficit that results in the loss of SC neurons with frontal RFs and, consequently, an absence of a topographic map of auditory space. These findings stress the importance of high-frequency hearing to compute spatially restricted receptive fields and serve as a caution to researchers that doing auditory-related research using the C57BL/6 genetic background may not be representative of true wild-type mice.

The effects of spatial attention and population receptive field size estimation on fMRI topographic mapping signals

The effects of spatial attention and population receptive field size estimation on fMRI topographic mapping signals

Dynamic stimulation evokes spatially focused receptive fields in bat auditory cortex

Bats can orient and hunt for prey in complete darkness using echolocation. Due to the pulse-like character of call emission they receive a stroboscopic view of their environment. During target approach, bats adjust their emitted echolocation calls to the specific requirements of the dynamically changing environmental and behavioral context. In addition to changes of the spectro-temporal call features, the spatial focusing of the beam of the sonar emissions onto the target is a conspicuous feature during target tracking. The neural processes underlying the complex sensory-motor interactions during target tracking are not well understood. In this study, we used a two-tone-pulse paradigm with 81 combinations of inter-aural intensity differences and six inter-pulse intervals in a passive hearing task to tackle the question of how transient changes in the azimuthal position of successive sounds are encoded by neurons in the auditory cortex of the bat Phyllostomus discolor. In a population of cortical neurons (11%, 24 of 217), spatial receptive fields were focused to a small region of frontal azimuthal positions during dynamic stimulation with tone-pulse pairs at short inter-pulse intervals. The response of these neurons might be important for the behaviorally observed locking of the sonar beam onto a selected target during the later stages of target tracking. Most interestingly, the majority of these neurons (88%, 21 of 24) were located in the posterior dorsal part of the auditory cortex. This cortical subfield might thus be specifically involved in the analysis of dynamic acoustic scenes.

All bits are welcome: a coherent model for synergistic generalization and discrimination

Event Abstract Back to Event All bits are welcome: a coherent model for synergistic generalization and discrimination András Lőrincz1* and Gábor Szirtes1 1 Eötvös Loránd University, Software Technology and Methodology, Hungary In modeling neural information processing, the ability to generalize and to discriminate are two desirable features, yet they seem to be in conflict. Based on recent results in signal processing we propose a two-stage representational model that learns to decompose the observations into a typical part serving generalization and a sparse form using an overcomplete dictionary. The latter form is useful in grasping peculiarities in a concise way, but even small perturbations can result in large variations. When these two representational forms are tuned in concert then both become more powerful: generalization gets more robust against outliers, while sparse coding can develop increasingly sparse overcomplete representations less affected by noise. On natural image patches our model simultaneously develops Difference-of-Gaussian like basis as preprocessing for the representation of the typicalities and Gabor filter like basis for representing sparse features of the inputs. In turn, the proposed model may provide a simple and coherent account on the hierarchical arrangement of the spatial receptive fields at the first stages of early visual processing (that is in the retina, LGN and primary visual cortex). The iterative algorithm can be implemented in a generative neural network form and it can be seen as a generalization and unification of other attractive proposals, like infomax, slow-feature analysis and sparse coding. We argue that the principles behind the proposed algorithm make it a central component in diverse hierarchical computations performed by neural systems. Keywords: Information Processing*, Learning, Memory, modelling, sensory processing Conference: Bernstein Conference on Computational Neuroscience, Berlin, Germany, 27 Sep - 1 Oct, 2010. Presentation Type: Presentation Topic: Bernstein Conference on Computational Neuroscience Citation: Lőrincz A and Szirtes G (2010). All bits are welcome: a coherent model for synergistic generalization and discrimination. Front. Comput. Neurosci. Conference Abstract: Bernstein Conference on Computational Neuroscience. doi: 10.3389/conf.fncom.2010.51.00012 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 01 Sep 2010; Published Online: 22 Sep 2010. * Correspondence: Dr. András Lőrincz, Eötvös Loránd University, Software Technology and Methodology, Budapest, Hungary, andras.lorincz@t-online.hu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers András Lőrincz Gábor Szirtes Google András Lőrincz Gábor Szirtes Google Scholar András Lőrincz Gábor Szirtes PubMed András Lőrincz Gábor Szirtes Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Multiplicative auditory spatial receptive fields created by a hierarchy of population codes.

A multiplicative combination of tuning to interaural time difference (ITD) and interaural level difference (ILD) contributes to the generation of spatially selective auditory neurons in the owl's midbrain. Previous analyses of multiplicative responses in the owl have not taken into consideration the frequency-dependence of ITD and ILD cues that occur under natural listening conditions. Here, we present a model for the responses of ITD- and ILD-sensitive neurons in the barn owl's inferior colliculus which satisfies constraints raised by experimental data on frequency convergence, multiplicative interaction of ITD and ILD, and response properties of afferent neurons. We propose that multiplication between ITD- and ILD-dependent signals occurs only within frequency channels and that frequency integration occurs using a linear-threshold mechanism. The model reproduces the experimentally observed nonlinear responses to ITD and ILD in the inferior colliculus, with greater accuracy than previous models. We show that linear-threshold frequency integration allows the system to represent multiple sound sources with natural sound localization cues, whereas multiplicative frequency integration does not. Nonlinear responses in the owl's inferior colliculus can thus be generated using a combination of cellular and network mechanisms, showing that multiple elements of previous theories can be combined in a single system.

Asymmetrical representation of auditory space in human cortex

Recent single-neuron recordings in monkeys and magnetoencephalography (MEG) data on humans suggest that auditory space is represented in cortex as a population rate code whereby spatial receptive fields are wide and centered at locations to the far left or right of the subject. To explore the details of this code in the human brain, we conducted an MEG study utilizing realistic spatial sound stimuli presented in a stimulus-specific adaptation paradigm. In this paradigm, the spatial selectivity of cortical neurons is measured as the effect the location of a preceding adaptor has on the response to a subsequent probe sound. Two types of stimuli were used: a wideband noise sound and a speech sound. The cortical hemispheres differed in the effects the adaptors had on the response to a probe sound presented in front of the subject. The right-hemispheric responses were attenuated more by an adaptor to the left than by an adaptor to the right of the subject. In contrast, the left-hemispheric responses were similarly affected by adaptors in these two locations. When interpreted in terms of single-neuron spatial receptive fields, these results support a population rate code model where neurons in the right hemisphere are more often tuned to the left than to the right of the perceiver while in the left hemisphere these two neuronal populations are of equal size.

Spatial receptive field organization of multisensory neurons and its impact on multisensory interactions

Previous work has established that the spatial receptive fields (SRFs) of multisensory neurons in the cerebral cortex are strikingly heterogeneous, and that SRF architecture plays an important deterministic role in sensory responsiveness and multisensory integrative capacities. The initial part of this contribution serves to review these findings detailing the key features of SRF organization in cortical multisensory populations by highlighting work from the cat anterior ectosylvian sulcus (AES). In addition, we have recently conducted parallel studies designed to examine SRF architecture in the classic model for multisensory studies, the cat superior colliculus (SC), and we present some of the preliminary observations from the SC here. An examination of individual SC neurons revealed marked similarities between their unisensory (i.e., visual and auditory) SRFs, as well as between these unisensory SRFs and the multisensory SRF. Despite these similarities within individual neurons, different SC neurons had SRFs that ranged from a single area of greatest activation (hot spot) to multiple and spatially discrete hot spots. Similar to cortical multisensory neurons, the interactive profile of SC neurons was correlated strongly to SRF architecture, closely following the principle of inverse effectiveness. Thus, large and often superadditive multisensory response enhancements were typically seen at SRF locations where visual and auditory stimuli were weakly effective. Conversely, subadditive interactions were seen at SRF locations where stimuli were highly effective. Despite the unique functions characteristic of cortical and subcortical multisensory circuits, our results suggest a strong mechanistic interrelationship between SRF microarchitecture and integrative capacity.

Auditory Spatial Tuning at the Crossroads of the Midbrain and Forebrain

The barn owl's midbrain and forebrain contain neurons tuned to sound direction. The spatial receptive fields of these neurons result from sensitivity to combinations of interaural time (ITD) and level (ILD) differences over a broad frequency range. While a map of auditory space has been described in the midbrain, no similar topographic representation has been found in the forebrain. The first nuclei that belong exclusively to the forebrain and midbrain pathways are the thalamic nucleus ovoidalis (Ov) and the external nucleus of the inferior colliculus (ICx), respectively. The midbrain projects to the auditory thalamus before sharp spatial receptive fields emerge; although Ov and ICx receive projections from the same midbrain nuclei, they are not directly connected. We compared the spatial tuning in Ov and ICx. Thalamic neurons respond to a broader frequency range and their ITD and ILD tuning varied more across frequency. However, neurons in Ov showed spatial receptive fields as selective as neurons in ICx. Thalamic spatial receptive fields were tuned to frontal and contralateral space and correlated with their tuning to ITD and ILD. Our results indicate that spatial tuning emerges in both pathways by similar combination selectivity to ITD and ILD. However, the midbrain and the thalamus do not appear to repeat exactly the same processing, as indicated by the difference in frequency range and the broader tuning to binaural cues. The differences observed at the initial stages of these sound-localization pathways may reflect diverse functions and coding schemes of midbrain and forebrain.

Edge-Detection Filter Improves Spatial Resolution in the Electrosensory System of the Paddlefish

In many fishes, prey capture is guided primarily by vision. In the paddlefish, the electrosense can completely substitute for the visual system to detect tiny daphnia, their primary prey. Electroreceptors are distributed over the entire rostrum, head, and gill covers, and there are no accessory structures like a lens to form an image. To accurately locate planktonic prey in three-dimensional space, the poor spatial resolving power of peripheral receptors has to be improved by another mechanism. We have investigated information processing in the electrosensory system of the paddlefish at hind- and midbrain levels by recording single cells extracellularly. We stimulated with a linear array of electrodes that simulated a moving dipole field. In addition, global electric fields were applied to simulate the temporal component of a moving dipole only. Some stimulation were done with sinusoidal fields. The fire rate of cells in the hindbrain followed the first derivative of the stimulus wave form. In contrast, the response of tectal cells were similar to the third derivative. This improves spatial resolution and receptive fields of tectal units are much smaller than the ones of hind brain units. The principle is similar to a Laplacian of Gaussian filter that is commonly used in digital image processing. However, instead of working in the space domain, the paddlefish edge detection filter works in the time domain, thus eliminating the need for extensive interconnections in an array of topographically organized neurons.

Some brain mechanisms for auditory scene analysis.

Detection and discrimination of sounds in complex auditory environments is facilitated by knowledge of the locations of sound sources and by spatial separation of signals and maskers. In studies of the spatial sensitivity of auditory cortical neurons in anesthetized cats, we find that location‐dependent variation in temporal spike patterns can signal sound‐source locations throughout up to 360 deg of space. In awake cats, spatial receptive fields can be similarly broad, although cortical neurons show a greater diversity of temporal patterns than is seen in anesthetized conditions. Our recent results demonstrate that spatial tuning tends to sharpen when a cat is engaged in an auditory task, compared to idle conditions, and that tuning can sharpen even further when the task involves sound localization. In human psychophysical studies, spatial hearing permits perceptual “stream segregation” of multiple temporally interleaved sequences of sounds. In parallel studies of the auditory cortex in anesthetized cats...

"I look in your eyes, honey": internal face features induce spatial frequency preference for human face processing.

Numerous psychophysical experiments found that humans preferably rely on a narrow band of spatial frequencies for recognition of face identity. A recently conducted theoretical study by the author suggests that this frequency preference reflects an adaptation of the brain's face processing machinery to this specific stimulus class (i.e., faces). The purpose of the present study is to examine this property in greater detail and to specifically elucidate the implication of internal face features (i.e., eyes, mouth, and nose). To this end, I parameterized Gabor filters to match the spatial receptive field of contrast sensitive neurons in the primary visual cortex (simple and complex cells). Filter responses to a large number of face images were computed, aligned for internal face features, and response-equalized (“whitened”). The results demonstrate that the frequency preference is caused by internal face features. Thus, the psychophysically observed human frequency bias for face processing seems to be specifically caused by the intrinsic spatial frequency content of internal face features.

Environmental enrichment improves behavioral performance and auditory spatial representation of primary auditory cortical neurons in rat

Environmental enrichment (EE) has an important role in brain plasticity. Early research has shown that EE increases the response strength of neurons in the auditory cortex, but it remains unknown whether EE can influence the process of spatial localization in the auditory system. In this study, we raised rats in enriched and standard conditions from postnatal day 10 to day 56. By testing behavioral tasks via auditory cues, we have shown that EE improved the number of correct scores, but decreased the reaction time and azimuth deviation in behavioral performance of sound-azimuth discrimination. By in vivo extracellular recording, we have shown that EE enhanced the directional sensitivity of neurons in the primary auditory cortex. For example, EE rats had a smaller spatial receptive field, sharper frequency tuning curve and directional selective curve of auditory neurons compared with normal rats. Our findings indicate that early exposure to EE increases directional sensitivity. These results provide an insight into developmental plasticity in the auditory system.

Virtual Adult Ears Reveal the Roles of Acoustical Factors and Experience in Auditory Space Map Development

Auditory neurons in the superior colliculus (SC) respond preferentially to sounds from restricted directions to form a map of auditory space. The development of this representation is shaped by sensory experience, but little is known about the relative contribution of peripheral and central factors to the emergence of adult responses. By recording from the SC of anesthetized ferrets at different age points, we show that the map matures gradually after birth; the spatial receptive fields (SRFs) become more sharply tuned and topographic order emerges by the end of the second postnatal month. Principal components analysis of the head-related transfer function revealed that the time course of map development is mirrored by the maturation of the spatial cues generated by the growing head and external ears. However, using virtual acoustic space stimuli, we show that these acoustical changes are not by themselves responsible for the emergence of SC map topography. Presenting stimuli to infant ferrets through virtual adult ears did not improve the order in the representation of sound azimuth in the SC. But by using linear discriminant analysis to compare different response properties across age, we found that the SRFs of infant neurons nevertheless became more adult-like when stimuli were delivered through virtual adult ears. Hence, although the emergence of auditory topography is likely to depend on refinements in neural circuitry, maturation of the structure of the SRFs (particularly their spatial extent) can be largely accounted for by changes in the acoustics associated with growth of the head and ears.

Direction Selectivity of Neurons in the Macaque Lateral Intraparietal Area

The lateral intraparietal area (LIP) of the macaque is believed to play a role in the allocation of attention and the plan to make saccadic eye movements. Many studies have shown that LIP neurons generally encode the static spatial location demarked by the receptive field (RF). LIP neurons might also provide information about the features of visual stimuli within the RF. For example, LIP receives input from cortical areas in the dorsal visual pathway that contain many direction-selective neurons. Here we examine direction selectivity of LIP neurons. Animals were only required to fixate while motion stimuli appeared in the RF. To avoid spatial confounds, the motion stimuli were patches of randomly arrayed dots that moved with 100% coherence in eight different directions. We found that the majority (61%) of LIP neurons were direction selective. The direction tuning was fairly broad, with a median direction-tuning bandwidth of 136 degrees. The average strength of direction selectivity was weaker in LIP than that of other areas of the dorsal visual stream but that difference may be because of the fact that LIP neurons showed a tonic offset in firing whenever a visual stimulus was in the RF, independent of direction. Direction-selective neurons do not seem to constitute a functionally distinct subdivision within LIP, because those neurons had robust, sustained delay-period activity during a memory delayed saccade task. The direction selectivity could also not be explained by asymmetries in the spatial RF, in the hypothetical case that the animals attended to slightly different locations depending on the direction of motion in the RF. Our results show that direction selectivity is a distinct attribute of LIP neurons in addition to spatial encoding.

Unmasking the CA1 Ensemble Place Code by Exposures to Small and Large Environments: More Place Cells and Multiple, Irregularly Arranged, and Expanded Place Fields in the Larger Space

In standard experimental environments, a constant proportion of CA1 principal cells are place cells, each with a spatial receptive field called a place field. Although the properties of place cells are a basis for understanding the mammalian representation of spatial knowledge, there is no consensus on which of the two fundamental neural-coding hypotheses correctly accounts for how place cells encode spatial information. Within the dedicated-coding hypothesis, the current activity of each cell is an independent estimate of the location with respect to its place field. The average of the location estimates from many cells represents current location, so a dedicated place code would degrade if single cells had multiple place fields. Within the alternative, ensemble-coding hypothesis, the concurrent discharge of many place cells is a vector that represents current location. An ensemble place code is not degraded if single cells have multiple place fields as long as the discharge vector at each location is unique. Place cells with multiple place fields might be required to represent the substantially larger space in more natural environments. To distinguish between the dedicated-coding and ensemble-coding hypotheses, we compared the characteristics of CA1 place fields in a standard cylinder and an approximately six times larger chamber. Compared with the cylinder, in the chamber, more CA1 neurons were place cells, each with multiple, irregularly arranged, and enlarged place fields. The results indicate that multiple place fields is a fundamental feature of CA1 place cell activity and that, consequently, an ensemble place code is required for CA1 discharge to accurately signal location.

Context-Dependent Changes in Functional Circuitry in Visual Area MT

Animals can flexibly change their behavior in response to a particular sensory stimulus; the mapping between sensory and motor representations in the brain must therefore be flexible as well. Changes in the correlated firing of pairs of neurons may provide a metric of changes in functional circuitry during behavior. We studied dynamic changes in functional circuitry by analyzing the noise correlations of simultaneously recorded MT neurons in two behavioral contexts: one that promotes cooperative interactions between the two neurons and another that promotes competitive interactions. We found that identical visual stimuli give rise to differences in noise correlation in the two contexts, suggesting that MT neurons receive inputs of central origin whose strength changes with the task structure. The data are consistent with a mixed feature-based attentional strategy model in which the animal sometimes alternates attention between opposite directions of motion and sometimes attends to the two directions simultaneously.

Clustering of Self-Motion Selectivity and Visual Response Properties in Macaque Area MSTd

Neurons in the dorsal subdivision of the medial superior temporal area (MSTd) show directionally selective responses to both visual (optic flow) and vestibular stimuli that correspond to translational or rotational movements of the subject. Previous work has shown that MSTd neurons are clustered within the cortex according to their directional preferences for optic flow, suggesting that there may be a topographic mapping of self-motion vectors in MSTd. If MSTd provides a multisensory representation of self-motion information, then MSTd neurons may also be expected to show clustering according to their directional preferences for vestibular signals, but this has not been tested previously. We have examined clustering of vestibular signals by comparing the tuning of isolated single units (SUs) with the undifferentiated multiunit (MU) activity of several neighboring neurons recorded from the same microelectrode. We find that directional preferences for both translational and rotational vestibular stimuli, like those for optic flow, are clustered within area MSTd. MU activity often shows significant tuning for vestibular stimuli, although this MU selectivity is generally weaker for translation than for rotation. When directional tuning is observed in MU activity, the direction preference generally agrees closely with that of a simultaneously recorded SU. We also examined clustering of visual receptive field properties in MSTd by analyzing receptive field maps obtained using a reverse-correlation technique. We find that both the local directional preferences and overall spatial receptive field profiles are well clustered in MSTd. Overall, our findings have implications for how visual and vestibular signals regarding self-motion may be decoded from populations of MSTd neurons.