Role In Visual Processing Research Articles

Modulation of microsaccade rate by task difficulty revealed through between- and within-trial comparisons

Microsaccades (MSs) are small eye movements that occur during attempted visual fixation. While most studies concerning MSs focus on their roles in visual processing, some also suggest that the MS rate can be modulated by the amount of mental exertion involved in nonvisual processing. The current study focused on the effects of task difficulty on MS rate in a nonvisual mental arithmetic task. Experiment 1 revealed a general inverse relationship between MS rate and subjective task difficulty. During Experiment 2, three task phases with different requirements were identified: during calculation (between stimulus presentation and response), postcalculation (after reporting an answer), and a control condition (undergoing a matching sequence of events without the need to make a calculation). MS rate was observed to approximately double from the during-calculation phase to the postcalculation phase, and was significantly higher in the control condition compared to postcalculation. Only during calculation was the MS rate generally decreased with greater task difficulty. Our results suggest that the nonvisual cognitive processing can suppress MS rate, and that the extent of such suppression is related to the task difficulty.

TRPM3 expression in mouse retina.

Transient receptor potential (TRP) channels constitute a large family of cation permeable ion channels that serve crucial functions in sensory systems by transducing environmental changes into cellular voltage and calcium signals. Within the retina, two closely related members of the melastatin TRP family, TRPM1 and TRPM3, are highly expressed. TRPM1 has been shown to be required for the depolarizing response to light of ON-bipolar cells, but the role of TRPM3 in the retina is unknown. Immunohistochemical staining of mouse retina with an antibody directed against the C-terminus of TRPM3 labeled the inner plexiform layer (IPL) and a subset of cells in the ganglion cell layer. Within the IPL, TRPM3 immunofluorescence was markedly stronger in the OFF sublamina than in the ON sublamina. Electroretinogram recordings showed that the scotopic and photopic a- and b-waves of TRPM3-/- mice are normal indicating that TRPM3 does not play a major role in visual processing in the outer retina. TRPM3 activity was measured by calcium imaging and patch-clamp recording of immunopurified retinal ganglion cells. Application of the TRPM3 agonist, pregnenolone sulfate (PS), stimulated increases in intracellular calcium in ~40% of cells from wild type and TRPM1‑/‑ mice, and the PS-stimulated increases in calcium were blocked by co-application of mefenamic acid, a TRPM3 antagonist. No PS-stimulated changes in fluorescence were observed in ganglion cells from TRPM3-/- mice. Similarly, PS-stimulated currents that could be blocked by mefenamic acid were recorded from wild type retinal ganglion cells but were absent in ganglion cells from TRPM3-/- mice.

The role of visual processing in motor learning and control: Insights from electroencephalography

Traditionally our understanding of goal-directed action been derived from either behavioral findings or neuroanatomically derived imaging (i.e., fMRI). While both of these approaches have proven valuable, they lack the ability to determine a direct locus of function while concurrently having the necessary temporal precision needed to understand millisecond scale neural interactions respectively. In this review we summarize some seminal behavioral findings across three broad areas (target perturbation, feed-forward control, and feedback processing) and for each discuss the application of electroencephalography (EEG) to the understanding of the temporal nature of visual cue utilization during movement planning, control, and learning using four existing scalp potentials. Specifically, we examine the appropriateness of using the N100 potential as an indicator of corrective behaviors in response to target perturbation, the N200 as an index of movement planning, the P300 potential as a metric of feed-forward processes, and the feedback-related negativity as an index of motor learning. Although these existing components have potential for insight into cognitive contributions and the timing of the neural processes that contribute to motor control further research is needed to expand the control-related potentials and to develop methods to permit their accurate characterization across a wide range of behavioral tasks.

Retinal Input Directs the Recruitment of Inhibitory Interneurons into Thalamic Visual Circuits

Inhibitory interneurons (INs) critically control the excitability and plasticity of neuronal networks, but whether activity can direct INs into specific circuits during development is unknown. Here, we report that in the dorsal lateral geniculate nucleus (dLGN), which relays retinal input to the cortex, circuit activity is required for the migration, molecular differentiation, and functional integration of INs. We first characterize the prenatal origin and molecular identity of dLGN INs, revealing their recruitment from an Otx2(+) neuronal pool located in the adjacent ventral LGN. Using time-lapse and electrophysiological recordings, together with genetic and pharmacological perturbation of retinal waves, we show that retinal activity directs the navigation and circuit incorporation of dLGN INs during the first postnatal week, thereby regulating the inhibition of thalamocortical circuits. These findings identify an input-dependent mechanism regulating IN migration and circuit inhibition, which may account for the progressive recruitment of INs into expanding excitatory circuits during evolution.

Visual callosal connections: role in visual processing in health and disease

Visual cortical areas in the two sides of the brain are interconnected by interhemispheric fibers passing through the splenium of the corpus callosum. In this review, we summarize data concerning the anatomical features of visual callosal connections, their roles in basic visual processing, and how their alterations contribute to visual deficits in different human neuropathologies. Splenial fibers represent a population of excitatory axons with varying diameters, which interconnect cortical columns with similar functional properties (i.e., same orientation selectivity) in the two hemispheres. Their branches activate simultaneously distinct iso-oriented columns in the contralateral hemisphere, thus mediating forms of stimulus-dependent interhemispheric synchronization. Callosal branches also make synapses onto GABAergic cells, resulting in an inhibitory modulation of visual processing that involves both iso-oriented and cross-oriented cortical networks. Interhemispheric inhibition appears to predominate at short latencies following callosal activation, whereas excitation becomes more robust with increasing delays. These callosal effects are dynamically adapted to the incoming visual activity, so that stimuli providing only weak afferent input are facilitated by callosal pathways, whereas strong visual input via the retinogeniculate pathway tends to be offset by transcallosal inhibition. We also review data highlighting the contribution of callosal input activity to maturation of visual function during early 'critical periods' in brain development and describe how interhemispheric transfer of visual information is rerouted in cases of callosal agenesis or following splenial damage. Finally, we provide an overview of alterations in splenium anatomy or function that may be at the basis of visual defects in several pathologic conditions.

Mapping of the Receptive Fields in the Optic Tectum of Chicken (Gallus gallus) Using Sparse Noise

The optic tectum plays a key role in visual processing in birds. While the input from the retina is topographic in the superficial layers, the deep layers project to the thalamic nucleus rotundus in a functional topographical manner. Although the receptive fields of tectal neurons in birds have been mapped before, a high resolution description of the white and black subfields of the receptive fields of tectal neurons is not available. We measured the receptive fields of neurons in the different layers of the tectum of anesthetized chickens with black and white stimuli that were flashed on a grey background in fast progression. Our results show that neurons in the deep layers of the optic tectum tend to respond stronger to black stimuli compared to white stimuli. In addition, the receptive field sizes are larger when measured using black stimuli than with white stimuli. While the black subfield was significantly larger than the white subfield for the intermediate and deep layers, no significant effects were found for the superficial layers. Finally, we investigated the optimal stimulus size in a subset of the neurons and found that these cells respond best to small white stimuli and to large black stimuli. In the majority of the cases the response was stronger to a large black bar than to a small white bar. We propose that such a stronger response to black stimuli might be advantageous for the detection of darker objects against the brighter sky.

Coaxial Anisotropy of Cortical Point Spread in Human Visual Areas

A focal stimulus triggers neural activity that spreads to cortical regions far beyond the stimulation site, creating a so-called "cortical point spread" (CPS). Animal studies found that V1 neurons possess lateral connections with neighboring neurons that prefer similar orientations and to neurons representing visuotopic regions that are constrained by their preferred orientation axis. Although various roles in visual processing are proposed for this anatomical anisotropy of lateral connections, evidence for a corresponding "functional" anisotropy in CPS is lacking or inconsistent in animal studies and absent in humans. To explore functional anisotropy, we inspected axial constraints on CPS in human visual cortex using functional magnetic resonance imaging. We defined receptive fields (RFs) of unit gray matter volumes and delineated the spatial extents of CPS in visuotopic space. The CPS triggered by foveal stimuli exhibited coaxial anisotropy with larger spatial extents along the axis of stimulus orientation. Furthermore, the spatial extents of CPS along the coaxial direction increased with an increasing similarity of local sites to the CPS-inducing stimulus in orientation preference. From CPS driven by multifocal stimuli, the coaxially biased spread was also found in cortical regions in the periphery, albeit reduced in degree, and was invariant to a varying degree of radial relationship between stimuli and RF positions of local sites, rejecting radial bias as an origin of coaxial anisotropy. Our findings provide a bridge between the anatomical anisotropy seen in animal visual cortex and a possible network property supporting spatial contextual effects in human visual perception.

Resveratrol Inhibits GABACρ Receptor-Mediated Ion Currents Expressed inXenopusOocytes

Resveratrol is a phytoalexin found in grapes, red wine, and berries. Resveratrol has been known to have many beneficial health effects, such as anti-cancer, neuroprotective, anti-inflammatory, and life-prolonging effects. However, relatively little is known about the effects of resveratrol on the regulation of ligand-gated ion channels. We have previously reported that resveratrol regulates subsets of homomeric ligand-gated ion channels such as those of 5-HT3A receptors. The γ-aminobutyric acidC (GABAC) receptor is mainly expressed in retinal bipolar cells and plays an important role in visual processing. In the present study, we examined the effects of resveratrol on the channel activity of homomeric GABAC receptor expressed in Xenopus oocytes injected with cRNA encoding human GABAC ρ subunits. Our data show that the application of GABA elicits an inward peak current (IGABA) in oocytes that express the GABAC receptor. Resveratrol treatment had no effect on oocytes injected with H2O or with GABAC receptor cRNA. Co-treatment with resveratrol and GABA inhibited IGABA in oocytes with GABAC receptors. The inhibition of IGABA by resveratrol was in a reversible and concentration-dependent manner. The IC50 of resveratrol was 28.9±2.8 µM in oocytes expressing GABAC receptor. The inhibition of IGABA by resveratrol was in voltage-independent and non-competitive manner. These results indicate that resveratrol might regulate GABAC receptor expression and that this regulation might be one of the pharmacological actions of resveratrol on the nervous system.

The Role of Visual Processing in Learning to Read Chinese Characters

This study investigated the development of two types of visual processing, visual processing of geometric figures (geometric-figure processing) and visual processing of character configurations (character-configuration processing), and their respective contributions to Chinese reading. Participants were 116 Chinese children from junior kindergarten to Grade 2. The development of geometric-figure processing was not related to literacy experience, as there was little improvement from senior kindergarten to Grade 1. In contrast, children in Grade 1 outperformed those in senior kindergarten on character-configuration processing. With respect to the relationship with character reading, geometric-figure processing predicted reading only in kindergarten, whereas character-configuration processing explained unique variance in reading in both kindergarten and primary grades. The results suggest that the two types of visual processing skills may have different underlying mechanisms. Character-configuration processing is more closely related to character reading than geometric-figure processing.

General anesthesia as a possible GABAergic modulator affects visual processing in children

Gamma-Aminobutyric Acid (GABA) inhibitory interneurons play an important role in visual processing, as is revealed by studies administering drugs in human and monkey adults. Investigating this process in children requires different methodologies, due to ethical considerations. The current study aimed to investigate whether a new method, being general anesthesia using Sevoflurane, can be used to trace the effects of GABAergic modulation on visual brain functioning in children. To this aim, visual processing was investigated in children aged 4–12 years who were scheduled for minor urologic procedures under general anesthesia in day-care treatment. In a visual segmentation task, the difference in Event-Related Potential (ERP) response to homogeneous and textured stimuli was investigated. In addition, psychophysical performance on visual acuity and contrast sensitivity were measured. Results were compared between before and shortly after anesthesia. In two additional studies, effects at 1 day after anesthesia and possible effects of task-repetition were investigated. ERP results showed longer latency and lower amplitude of the Texture Negativity (TN) component shortly after compared to before anesthesia. No effects of anesthesia on psychophysical measurements were found. No effects at 1 day after anesthesia or of repetition were revealed either. These results show that GABAergic modulation through general anesthesia affects ERP reflections of visual segmentation in a similar way in children as benzodiazepine does in adults, but that effects are not permanent. This demonstrates that ERP measurement after anesthesia is a successful method to study effects of GABAergic modulation in children.

The Frequency of Visually Induced Gamma-Band Oscillations Depends on the Size of Early Human Visual Cortex

The structural and functional architecture of the human brain is characterized by considerable variability, which has consequences for visual perception. However, the neurophysiological events mediating the relationship between interindividual differences in cortical surface area and visual perception have, until now, remained unknown. Here, we show that the retinotopically defined surface areas of central V1 and V2 are correlated with the peak frequency of visually induced oscillations in the gamma band, as measured with magnetoencephalography. Gamma-band oscillations are thought to play an important role in visual processing. We propose that individual differences in macroscopic gamma frequency may be attributed to interindividual variability in the microscopic architecture of visual cortex.

Intrinsic properties and functional circuitry of the AII amacrine cell

Amacrine cells represent the most diverse class of retinal neuron, comprising dozens of distinct cell types. Each type exhibits a unique morphology and generates specific visual computations through its synapses with a subset of excitatory interneurons (bipolar cells), other amacrine cells, and output neurons (ganglion cells). Here, we review the intrinsic and network properties that underlie the function of the most common amacrine cell in the mammalian retina, the AII amacrine cell. The AII connects rod and cone photoreceptor pathways, forming an essential link in the circuit for rod-mediated (scotopic) vision. As such, the AII has become known as the rod-amacrine cell. We, however, now understand that AII function extends to cone-mediated (photopic) vision, and AII function in scotopic and photopic conditions utilizes the same underlying circuit: AIIs are electrically coupled to each other and to the terminals of some types of ON cone bipolar cells. The direction of signal flow, however, varies with illumination. Under photopic conditions, the AII network constitutes a crossover inhibition pathway that allows ON signals to inhibit OFF ganglion cells and contributes to motion sensitivity in certain ganglion cell types. We discuss how the AII's combination of intrinsic and network properties accounts for its unique role in visual processing.

Whole brain recordings reveal physiologically separable endogenous oscillations in Drosophila melanogaster

Event Abstract Back to Event Whole brain recordings reveal physiologically separable endogenous oscillations in Drosophila melanogaster Angelique C. Paulk1* and Bruno Van Swinderen1 1 Queensland Brain Institute, University of Queensland, Queensland Brain Institute, Australia By virtue of their connectivity and underlying physiological properties, neural networks exhibit endogenous oscillations. Bands of endogenous oscillatory activity have been associated with attention, sensory processing, sleep, and place memory in mammals. In addition, specific brain areas produce area-specific oscillations in select frequency bands, such as the prevalence of the theta rhythm in the hippocampus or the gamma rhythm in the cortex. To explore the functional relationship between endogenous oscillations, their underlying neural circuitry, and behavior, we developed a multichannel recording preparation to use in the fly, Drosophila melanogaster (Figure 1). We recorded local field potentials (LFP) and the neural population activity (through multiunit activity) from primary visual processing centers along with central, higher order, brain regions simultaneously. We found that fly brain activity exhibited statistically separable frequency domains of endogenous activity, both in terms of LFP and spiking multiunit activity, which varied across the brain regions. When we mapped the recording sites to specific brain regions, we found central brain regions exhibited more slow-wave oscillations while the primary visual processing centers had significantly higher 15-80 Hz oscillations. To determine a functional role of these frequency bands, and their potential role in visual processing, we used flickering light stimuli and found that the brain regions had a frequency-dependent response to the flickering light. Finally, by transiently activating specific neural circuits, we found that the amplitudes of the oscillations in these frequency bands could be independently altered by increasing the activity of either excitatory or inhibitory circuits. This effect was independent of fly movements, indicating that the oscillatory activity we observe correlates with the underlying neural activity. Figure 1. Multichannel recording from a Drosophila brain can be mapped to specific brain regions. B. Recordings from different brain regions reveal endogenous oscillations which can be subdivided into frequency bands (C). Figure 1 Acknowledgements The authors would like to thank members of the van Swinderen laboratory for feedback. Funding was provided by the Queensland Brain Institute and the Australian Research Council. Keywords: central complex, local field potential, multiunit activity, optic lobe, Vision Conference: Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012. Presentation Type: Poster (but consider for Participant Symposium) Topic: Novel Tools and Methods Citation: Paulk AC and Van Swinderen B (2012). Whole brain recordings reveal physiologically separable endogenous oscillations in Drosophila melanogaster. Conference Abstract: Tenth International Congress of Neuroethology. doi: 10.3389/conf.fnbeh.2012.27.00225 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: 30 Apr 2012; Published Online: 07 Jul 2012. * Correspondence: Dr. Angelique C Paulk, Queensland Brain Institute, University of Queensland, Queensland Brain Institute, Brisbane, Australia, acpaulk13@gmail.com 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 Angelique C Paulk Bruno Van Swinderen Google Angelique C Paulk Bruno Van Swinderen Google Scholar Angelique C Paulk Bruno Van Swinderen PubMed Angelique C Paulk Bruno Van Swinderen 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.

Pulsed Out of Awareness: EEG Alpha Oscillations Represent a Pulsed-Inhibition of Ongoing Cortical Processing

Alpha oscillations are ubiquitous in the brain, but their role in cortical processing remains a matter of debate. Recently, evidence has begun to accumulate in support of a role for alpha oscillations in attention selection and control. Here we first review evidence that 8–12 Hz oscillations in the brain have a general inhibitory role in cognitive processing, with an emphasis on their role in visual processing. Then, we summarize the evidence in support of our recent proposal that alpha represents a pulsed-inhibition of ongoing neural activity. The phase of the ongoing electroencephalography can influence evoked activity and subsequent processing, and we propose that alpha exerts its inhibitory role through alternating microstates of inhibition and excitation. Finally, we discuss evidence that this pulsed-inhibition can be entrained to rhythmic stimuli in the environment, such that preferential processing occurs for stimuli at predictable moments. The entrainment of preferential phase may provide a mechanism for temporal attention in the brain. This pulsed inhibitory account of alpha has important implications for many common cognitive phenomena, such as the attentional blink, and seems to indicate that our visual experience may at least some times be coming through in waves.

Heterogeneous response dynamics in retinal ganglion cells: the interplay of predictive coding and adaptation.

To make efficient use of their limited signaling capacity, sensory systems often use predictive coding. Predictive coding works by exploiting the statistical regularities of the environment--specifically, by filtering the sensory input to remove its predictable elements, thus enabling the neural signal to focus on what cannot be guessed. To do this, the neural filters must remove the environmental correlations. If predictive coding is to work well in multiple environments, sensory systems must adapt their filtering properties to fit each environment's statistics. Using the visual system as a model, we determine whether this happens. We compare retinal ganglion cell dynamics in two very different environments: white noise and natural. Because natural environments have more power than that of white noise at low temporal frequencies, predictive coding is expected to produce a suppression of low frequencies and an enhancement of high frequencies, compared with the behavior in a white-noise environment. We find that this holds, but only in part. First, predictive coding behavior is not uniform: most on cells manifest it, whereas off cells, on average, do not. Overlaid on this nonuniformity between cell classes is further nonuniformity within both cell classes. These findings indicate that functional considerations beyond predictive coding play an important role in shaping the dynamics of sensory adaptation. Moreover, the differences in behavior between on and off cell classes add to the growing evidence that these classes are not merely homogeneous mirror images of each other and suggest that their roles in visual processing are more complex than expected from the classic view.

Modular organization in area 21a of the cat revealed by optical imaging: comparison with the primary visual cortex

Area 21a, located on the cat's lateral suprasylvian cortex, is considered as a higher-order cortical area. Little is known about its specific role in visual processing. In this study, the functional organization of area 21a was investigated by optical imaging of intrinsic signals and was compared to that of primary visual areas. We found a clear modular pattern for orientation selectivity in area 21a, with signal amplitude being four times lower than that in primary visual areas. There were no significant differences between the domains' characteristics, nor the tuning bandwidth, in areas of the primary visual cortex (17 and 18) and 21a. This suggests that the basic cortical structure is independent of the hierarchical level or function of one area. A uniform representation of spatial frequency was found in areas 17 and 18, as well as in area 21a. The mean preferred spatial frequency in area 21a was 0.30 c/deg. In contrast to area 18, no direction maps were observed in area 21a whether drifting gratings or random dot kinematograms were used. This study supports the proposal that area 21a plays a pivotal role along the ventral processing stream and is mainly involved in form processing.

The Role of Native-Language Phonology in the Auditory Word Identification and Visual Word Recognition of Russian–English Bilinguals

Does native language phonology influence visual word processing in a second language? This question was investigated in two experiments with two groups of Russian-English bilinguals, differing in their English experience, and a monolingual English control group. Experiment 1 tested visual word recognition following semantic categorization of words containing four phonological vowel contrasts (/i/-/u/,/I/-/A/,/i/-/I/,/epsilon/-/ae/). Experiment 2 assessed auditory identification accuracy of words containing these four contrasts. Both bilingual groups demonstrated reduced accuracy in auditory identification of two English vowel contrasts absent in their native phonology (/i/-/I/,epsilon/-/ae/). For late- bilinguals, auditory identification difficulty was accompanied by poor visual word recognition for one difficult contrast (/i/-/I/). Bilinguals' visual word recognition moderately correlated with their auditory identification of difficult contrasts. These results indicate that native language phonology can play a role in visual processing of second language words. However, this effect may be considerably constrained by orthographic systems of specific languages.

Neuromodulatory role of acetylcholine in visually-induced cortical activation: Behavioral and neuroanatomical correlates

Acetylcholine is released in the primary visual cortex during visual stimulation and may have a neuromodulatory role in visual processing. The present study uses both behavioral and functional neuroanatomy investigations to examine this role in the rat. In the first set of experiments the cholinergic system was lesioned with 192 immunoglobulin G (IgG) saporin and the visual acuity and performance in a visual water maze task were assessed. The cholinergic lesion did not affect the visual acuity measured pre- and post-lesion but it did reduce the efficiency to learn a novel orientation discrimination task measured post-lesion. In order to better understand the involvement of the cholinergic system in the neuronal activity in the visual cortex c-Fos expression induced by patterned visual stimulation was further investigated. Results obtained following lesion of the cholinergic fibers (192 IgG-saporin or quisqualic acid), muscarinic inhibition (scopolamine), or NMDA receptor inhibition (CPP) were compared with control conditions. Double and triple immunolabeling was used in order to determine the neurochemical nature of the activated cortical cells. The results demonstrated that patterned stimulation elicited a significant increase in c-Fos immunolabeled neurons in layer IV of the contralateral primary visual cortex to the stimulated eye which was completely abolished by cholinergic fibers lesion as well as scopolamine administration. This effect was independent of NMDA receptor transmission. The c-Fos activation was predominantly observed in the glutamatergic spiny stellate cells and less frequently in GABAergic interneurons. Altogether, these results demonstrate a strong involvement of the basal forebrain cholinergic system in the modulation of post-synaptic visual processing, which could be related to cognitive enhancement or attention during visual learning.

Bio‐avalibale zinc in the outer segments of short wavelength cones

Abstract Purpose: Zinc has been postulated to play a role in both normal and degenerative processes in the retina. Often these processes require zinc that is tightly bound to enzymes. However, new evidences indicate that the presence of bio‐available zinc is a better indicator for the involvemnt of zinc in retinal fuctions. Using autometallography on sectioned tissues suggested that bio‐available zinc is absent in photoreceptor outer segments. The purpose of this study was to re‐examine this observation using a zinc selective fluorescence probe and freshly dissected, flat mounted retinae . Methods: Following deep anesthesia and decapitation, dark‐adapted retinae were immediately dissected from rat, goldfish and chicken eyes and flat mounted with the photoreceptor up. The retinae were immediately labeled with the zinc‐specific fluorescent probe ZP1 (Neurobiotex, USA). Readily releasable zinc was visualized using fluorescent and confocal microscopy. Results: Only a subset of photoreceptor outer segments were labeled by ZP1 in rat retinae (>0.5 %). This labelling was localized to a subset of the cones. The cone localization of this labeling was confirmed by using goldfish retinae where the cones are readily distinguishable from rods morphologically. To identify the subtype of zinc postive cones we labeled chicken retinae where the characteristic oil droplets within the cones identify their sub‐type. Here zinc appeared to be localized to short wavelength cone outer segments. Conclusions: This is the first demonstration that bio‐available zinc is present in photoreceptor outer segments. The specific localization to short wavelength cones suggest that zinc plays a selective role in visual processing of short wavelength light.

The role of the thalamic reticular nucleus in visual processing

The thalamic reticular nucleus (TRN) is an integral part of the thalamic circuitry that mediates the transfer of information to the cortex in the major sensory pathways. Here, we consider its role in visual processing. The TRN provides an inhibitory, GABA-mediated input to relay cells in the thalamus. A key feature of TRN cells is that, because of the extent of their dendritic arborization, they have the capacity to collate visual information from the ascending relay cells and feedback axons from the cortex over a wide retinotopic area, but they feed it back to the thalamus in a narrowly focused retinotopic projection. The corticofugal input to TRN cells is mediated by a higher proportion of AMPA receptors than that to thalamic relay cells and produces sharp EPSCs. We suggest that this might bias the TRN cells to precisely timed, feature-driven feedback from the visual cortex. TRN cells are particularly well driven by either phasic or moving stimuli, so the influence on relay cells provides a strong, transient, inhibitory modulation of relay-cell activity. This might aid image segmentation and highlight responses to moving contours. Although the role of the TRN has been especially linked to selective attention, we suggest that, for vision, its function is linked more closely to the operation of the thalamocortico-thalamic loop as a whole, and that retinotopically selective attentional effects draw on the interaction with the cortical mechanism that influences TRN and relay-cell circuitry equally.