Cortical Area MT Research Articles

Remembering Visual Motion: Neural Correlates of Associative Plasticity and Motion Recall in Cortical Area MT

The pictorial content of visual memories recalled by association is embodied by neuronal activity at the highest processing stages of primate visual cortex. This activity is elicited by top-down signals from the frontal lobe and recapitulates the bottom-up pattern normally obtained by the recalled stimulus. To explore the generality and mechanisms of this phenomenon, we recorded motion-sensitive neurons at an early stage of cortical processing. After monkeys learned to associate directions of motion with static shapes, these neurons exhibited unprecedented selectivity for the shapes. This emergent shape selectivity reflects activation of neurons representing the motion stimuli recalled by association, and it suggests that recall-related activity may be a general feature of neurons in visual cortex.

Adaptive Surround Modulation in Cortical Area MT

Visual motion perception relies on two opposing operations: integration and segmentation. Integration overcomes motion ambiguity in the visual image by spatial pooling of motion signals, whereas segmentation identifies differences between adjacent moving objects. For visual motion area MT, previous investigations have reported that stimuli in the receptive field surround, which do not elicit a response when presented alone, can nevertheless modulate responses to stimuli in the receptive field center. The directional tuning of this "surround modulation" has been found to be mainly antagonistic and hence consistent with segmentation. Here, we report that surround modulation in area MT can be either antagonistic or integrative depending upon the visual stimulus. Both types of modulation were delayed relative to response onset. Our results suggest that the dominance of antagonistic modulation in previous MT studies was due to stimulus choice and that segmentation and integration are achieved, in part, via adaptive surround modulation.

A Fast Biologically Inspired Algorithm for Recurrent Motion Estimation

We have previously developed a neurodynamical model of motion segregation in cortical visual area V1 and MT of the dorsal stream. The model explains how motion ambiguities caused by the motion aperture problem can be solved for coherently moving objects of arbitrary size by means of cortical mechanisms. The major bottleneck in the development of a reliable biologically inspired technical system with real-time motion analysis capabilities based on this neural model is the amount of memory necessary for the representation of neural activation in velocity space. We propose a sparse coding framework for neural motion activity patterns and suggest a means by which initial activities are detected efficiently. We realize neural mechanisms such as shunting inhibition and feedback modulation in the sparse framework to implement an efficient algorithmic version of our neural model of cortical motion segregation. We demonstrate that the algorithm behaves similarly to the original neural model and is able to extract image motion from real world image sequences. Our investigation transfers a neuroscience model of cortical motion computation to achieve technologically demanding constraints such as real-time performance and hardware implementation. In addition, the proposed biologically inspired algorithm provides a tool for modeling investigations to achieve acceptable simulation time.

Deficits in Short-Latency Tracking Eye Movements after Chemical Lesions in Monkey Cortical Areas MT and MST

Past work has suggested that the medial superior temporal area (MST) is involved in the initiation of three kinds of eye movements at short latency by large-field visual stimuli. These eye movements consist of (1) version elicited by linear motion (the ocular following response), (2) vergence elicited by binocular parallax (the disparity vergence response), and (3) vergence elicited by global motion toward or away from the fovea (the radial-flow vergence response). We investigated this hypothesis by recording the effects of ibotenic acid injections in the superior temporal sulcus (STS) of both hemispheres in five monkeys. After the injections, all three kinds of eye movements were significantly impaired, with the magnitude of the impairments often showing a strong correlation with the extent of the morphological damage in the three subregions of the STS: dorsal MST on the anterior bank, lateral MST and middle temporal area on the posterior bank. However, the extent of the lesions in the three subregions often covaried, rendering it difficult to assess their relative contributions to the various deficits. The effects of the lesions on other aspects of oculomotor behavior that are known to be important for the normal functioning of the three tracking mechanisms (e.g., ocular stability, fixation disparity) were judged to be generally minor and to contribute little to the impairments. We conclude that, insofar as MST sustained significant damage in all injected hemispheres, our findings are consistent with the hypothesis that MST is a primary site for initiating all three visual tracking eye movements at ultra-short latencies.

Motion Integration by Neurons in Macaque MT Is Local, Not Global

Direction-selective neurons in primary visual cortex have small receptive fields that encode the motions of local features. These motions often differ from the motion of the object to which they belong and must therefore be integrated elsewhere. A candidate site for this integration is visual cortical area MT (V5), in which cells with large receptive fields compute the motion of patterns. Previous studies of motion integration in MT have used stimuli that fill the receptive field, and thus do not test whether motion information is really integrated across this whole area. For each MT neuron, we identified two regions ("patches") within the receptive field that were approximately equally effective in driving responses. We then measured responses to plaids whose component gratings overlapped within a patch, and compared them with responses to the same component gratings presented in separate patches. Cells that were selective for the direction of motion of the whole pattern when the gratings overlapped lost this selectivity when the gratings were separated and became selective instead for the direction of motion of the individual components. If MT cells simply pooled all of the inputs that endow them with a receptive field, they would encode all of the motions in the receptive field as belonging to a single object. Our results indicate instead that critical elements of the computations underlying pattern-direction selectivity in MT are done locally, on a scale smaller than the whole receptive field.

Cortical locus of coherent motion deficits in deaf poor readers

Samar and Parasnis [Samar, V. J., & Parasnis, I. (2005). Dorsal stream deficits suggest hidden dyslexia among deaf poor readers: correlated evidence from reduced perceptual speed and elevated coherent motion detection thresholds. Brain and Cognition, 58, 300–311.] reported that correlated measures of coherent motion detection and perceptual speed predicted reading comprehension in deaf young adults. Because deficits in coherent motion detection have been associated with dyslexia in the hearing population, and because coherent motion detection is strongly dependent on extrastriate cortical area MT, these results are consistent with the claim that hidden dyslexia occurs within the deaf population and is associated with deficits in MT. However, coherent motion detection can also be influenced by subcortical deficits in both magnocellular and parvocellular pathways. To confirm the putative cortical locus of coherent motion perception deficits, we measured contrast thresholds for detecting the direction of movement of drifting sine wave gratings in the same participant group as [Samar, V. J., & Parasnis, I. (2005). Dorsal stream deficits suggest hidden dyslexia among deaf poor readers: correlated evidence from reduced perceptual speed and elevated coherent motion detection thresholds. Brain and Cognition, 58, 300–311.], under stimulus conditions that selectively biased for input from the subcortical magnocellular and parvocellular pathways, respectively. Contrast thresholds were not related to reading comprehension performance under either the magnocellular or parvocellular conditions. Furthermore, the previously reported correlations among reading comprehension, coherent motion thresholds, and perceptual speed remained significant even after contrast thresholds and non-verbal IQ were controlled in partial correlation analyses. In addition, coherent motion detection thresholds were found to correlate specifically with a reading-IQ discrepancy score, one commonly used indicator of dyslexia. These results provide direct psychophysical evidence that the previously reported deficit in coherent motion detection in deaf poor readers does not involve subcortical pathway deficits, but rather is associated with a cortical deficit likely involving area MT. They also strengthen the argument for the existence of hidden dyslexia in the deaf adult population.

Attention: beyond neural response increases

Spatial attention has been shown to enhance visual processing by increasing the strength of neural responses. A new study suggests that in cortical area MT it also shifts neural receptive fields toward attended targets.

Dynamic shifts of visual receptive fields in cortical area MT by spatial attention

Voluntary attention is the top-down selection process that focuses cortical processing resources on the most relevant sensory information. Spatial attention--that is, selection based on stimulus position--alters neuronal responsiveness throughout primate visual cortex. It has been hypothesized that it also changes receptive field profiles by shifting their centers toward attended locations and by shrinking them around attended stimuli. Here we examined, at high resolution, receptive fields in cortical area MT of rhesus macaque monkeys when their attention was directed to different locations within and outside these receptive fields. We found a shift of receptive fields, even far from the current location of attention, accompanied by a small amount of shrinkage. Thus, already in early extrastriate cortex, receptive fields are not static entities but are highly modifiable, enabling the dynamic allocation of processing resources to attended locations and supporting enhanced perception within the focus of attention by effectively increasing the local cortical magnification.

Local Field Potential in Cortical Area MT: Stimulus Tuning and Behavioral Correlations

Low-frequency electrical signals like those that compose the local field potential (LFP) can be detected at substantial distances from their point of origin within the brain. It is thus unclear how useful the LFP might be for assessing local function, for example, on the spatial scale of cortical columns. We addressed this problem by comparing speed and direction tuning of LFPs obtained from middle temporal area MT with the tuning of multiunit (MU) activity recorded simultaneously. We found that the LFP can be well tuned for speed and direction and is highly correlated with that of MU activity, particularly for frequencies at and above the gamma band. LFP tuning is substantially poorer for lower frequencies, although tuning for direction extends to lower frequencies than does tuning for speed. Our data suggest that LFP signals at and above the gamma band reflect neural processing on the spatial scale of cortical columns, within a few hundred micrometers of the electrode tip. Consistent with this notion, we also found that frequencies at and above the gamma band measured during a speed discrimination task exhibit an effect known as "choice probability," which reveals a particularly close relationship between neural activity and behavioral choices. In the LFP, this signature of the perceptual choice comprises a shift in relative power from low-frequency bands (alpha and beta) to the gamma band. It remains to be determined how LFP choice probability, which is a temporal signature, is related to conventional choice probability effects observed in spike rates.

Sensitivity to optic flow in human cortical areas MT and MST

The macaque V5/MT complex comprises several sub-regions but little is known of their human homologues. We examined human V5/MT with fMRI in terms of specificity to optic flow stimuli, a key characteristic of macaque MST. Stimuli were large fields of moving dots, forming coherent global flow patterns. Random motion was used as a control. Retinotopic mapping was also conducted. The previously suggested existence of at least two distinct sub-regions, MT and MST, within the V5/MT complex was confirmed. Human MT is activated about equally by all moving dot patterns, including random motion, suggesting that it has little sensitivity to global flow structure. As previously described, this region shows strong signs of retinotopic organization and is only weakly activated by stimuli confined to the ipsilateral hemifield. In human MST, located immediately anterior to MT and strongly driven by ipsilateral stimuli, activation varies markedly with optic flow structure. The strongest activation is produced by complex flow that contains multiple flow components (expansion, contraction and rotation). Single components produce rather less response, while rigid translation and random motion produce less still. The results suggest that human MST is strongly specialized for encoding global flow properties, while human MT is less so.

An oblique effect for local motion: Psychophysics and natural movie statistics

Human perception of visual motion is thought to involve two stages--estimation of local motion (i.e., of small features) and global motion (i.e., of larger objects)--identified with cortical areas V1 and MT, respectively. We asked if poor discrimination of oblique compared to cardinal directions (the oblique effect for motion; OEM) reflects a deficit in local or in global motion processing. We used an equivalent noise (EN) paradigm--where one measures direction discrimination thresholds in the presence of directional variability--to quantify local and global limits. We report that the OEM diminishes with increasing directional variability, indicating that global motion processing (the number of local motion signals pooled) is equal across all directions and that the OEM is attributable to anisotropies in local motion processing. To investigate the origin of this effect, we measured local motion statistics from natural movies (filmed from the point of view of a walking observer). This analysis reveals that the distribution of local directional energy on the oblique directions tends to be broader, and frequently more asymmetric, than on the cardinal directions. If motion detectors are optimized to deal with our visual world then such anisotropies likely explain the local nature of the OEM.

Area MT Neurons Respond to Visual Motion Distant From Their Receptive Fields

Neurons in cortical area MT have localized receptive fields (RF) representing the contralateral hemifield and play an important role in processing visual motion. We recorded the activity of these neurons during a behavioral task in which two monkeys were required to discriminate and remember visual motion presented in the ipsilateral hemifield. During the task, the monkeys viewed two stimuli, sample and test, separated by a brief delay and reported whether they contained motion in the same or in opposite directions. Fifty to 70% of MT neurons were activated by the motion stimuli presented in the ipsilateral hemifield at locations far removed from their classical receptive fields. These responses were in the form of excitation or suppression and were delayed relative to conventional MT responses. Both excitatory and suppressive responses were direction selective, but the nature and the time course of their directionality differed from the conventional excitatory responses recorded with stimuli in the RF. Direction selectivity of the excitatory remote response was transient and early, whereas the suppressive response developed later and persisted after stimulus offset. The presence or absence of these unusual responses on error trials, as well as their magnitude, was affected by the behavioral significance of stimuli used in the task. We hypothesize that these responses represent top-down signals from brain region(s) accessing information about stimuli in the entire visual field and about the behavioral state of the animal. The recruitment of neurons in the opposite hemisphere during processing of behaviorally relevant visual signals reveals a mechanism by which sensory processing can be affected by cognitive task demands.

Erratum: Instructive signals for motor learning from visual cortical area MT

Nat. Neurosci. 8, 813–819 (2005). The upper trace in the right panel of Figure 2b on page 814 was mislabeled as '250 ms', when it should have read '200 ms'. The corrected figure is below.

Implicit Attentional Selection of Bound Visual Features

Traditionally, research on visual attention has been focused on the processes involved in conscious, explicit selection of task-relevant sensory input. Recently, however, it has been shown that attending to a specific feature of an object automatically increases neural sensitivity to this feature throughout the visual field. Here we show that directing attention to a specific color of an object results in attentional modulation of the processing of task-irrelevant and not consciously perceived motion signals that are spatiotemporally associated with this color throughout the visual field. Such implicit cross-feature spreading of attention takes place according to the veridical physical associations between the color and motion signals, even under special circumstances when they are perceptually misbound. These results imply that the units of implicit attentional selection are spatiotemporally colocalized feature clusters that are automatically bound throughout the visual field.

Instructive signals for motor learning from visual cortical area MT

Sensory error signals have long been proposed to act as instructive signals to guide motor learning. Here we have exploited the temporal specificity of learning in smooth pursuit eye movements and the well-defined anatomical structure of the neural circuit for pursuit to identify a part of sensory cortex that provides instructive signals for motor learning in monkeys. We show that electrical microstimulation in the motion-sensitive middle temporal area (MT) of extrastriate visual cortex instructs learning in smooth eye movements in a way that closely mimics the learning instructed by real visual motion. We conclude that MT provides instructive signals for motor learning in smooth pursuit eye movements under natural conditions, suggesting a similar role for sensory cortices in many kinds of learned behaviors.

Localization of the Human Cortical Visual Area MT Based on Computer Aided Histological Analysis

We describe human area MT histologically based on the observer independent analysis of cortical myeloarchiteture, multiple complementary staining techniques and 3-D reconstruction. The topography of an architectonic field that presented constant structural characteristics across specimens was studied in relation to the sulcal geography of the occipito-temporal region. Objective and semi-automated analysis of local microstructure revealed a distinct cortical architecture and matched topographically the localization of MT derived from functional imaging. MT was localized by the histotopographic method in relation to definite macroscopic landmarks. This study demonstrates a new set of distinguishing architectonic features of human MT that permit localization on structural grounds and suggests that the characteristic laminar structure of this area may be related to its unique pattern of connections and to its role in visual perception.

Lateral neural model of binocular rivalry.

This article introduces a two-dimensionally extended, neuron-based model for binocular rivalry. The basic block of the model is a certain type of astable multivibrator comprising excitatory and inhibitory neurons. Many of these blocks are laterally coupled on a medium range to provide a two-dimensional layer. Our model, like others, needs noise to reproduce typical stochastic oscillations. Due to its spatial extension, the noise has to be laterally correlated. When the contrast ratio of the pictures varies, their share of the perception time changes in a way that is known from comparable experimental data (Levelt, 1965; Mueller & Blake, 1989). This is a result of the lateral coupling and not a property of the single model block. The presentation of simple and suitable inhomogeneous stimuli leads to an easily describable perception of periodically moving pictures like propagating fronts or breathing spots. This suggests new experiments. Under certain conditions, a bifurcation from static to moving perceptions is predicted and may be checked and employed by future experiments. Recent "paradox" (Logothetis, 1999) observations of two different neuron classes in cortical areas MT (Logothetis & Schall, 1989) and V4 (Leopold & Logothetis, 1996), one that behaves alike under rivaling and nonrivaling conditions and another that drastically changes its behavior, are interpreted as being related to separate inhibitor neurons.

Two-dimensional map of direction selectivity in cortical visual area MT of Cebus monkey.

We studied the spatial organization of direction of motion in visual area MT of the Cebus apella monkey. We used arrays of 6 (700 micro m apart) parallel electrodes in penetrations tangential to the cortical layers to record multi-unit responses to moving bars, at 200 micro m steps. We determined the direction selectivity at each recording site. The data from single penetrations showed cyclic and gradual changes in the direction selectivity of clusters of cells, intermixed with abrupt 180 degrees discontinuities along the electrode track. In order to obtain maps of direction of motion selectivity, we examined the spatial distribution of direction of motion in MT and we applied a method to determine the location of the centers of radial arrangements of direction selectivity. This tangential organization is characterized by slow continuous changes in direction of motion, interrupted by discontinuities. The changes in direction selectivity are organized radially in a pinwheel fashion and in slabs of linear variation. The pinwheel arrangements have 800-1400 micro m in diameter. The size of the radial arrangement is comparable to the point image size in area MT at each eccentricity.

Neural correlates of structure-from-motion perception in macaque V1 and MT.

Structure-from-motion (SFM) is the perception of three-dimensional shape from motion cues. We used a bistable SFM stimulus, which can be perceived in one of two different ways, to study how neural activity in cortical areas V1 and MT is related to SFM perception. Monkeys performed a depth-order task, where they indicated in which direction the front surface of a rotating SFM cylinder display was moving. To prevent contamination of the neural data because of eye position effects, all experiments with significant effects of radius, vergence, and velocity were excluded. As expected, the activity of approximately 50% of neurons in V1 and approximately 80% of neurons in MT is affected by the stimulus. Furthermore, the activity of 20% of neurons in area V1 is modulated with the percept. This proportion is higher in MT, where the activity of >60% of neurons is modulated with the percept. In both areas, this perceptual modulation occurs only in neurons with activity that is also affected by the stimulus. The perceptual modulation is not correlated with neural tuning properties in area V1, but it is in area MT. Together, these results suggest that V1 is not directly involved in the generation of the SFM percept, whereas MT is. The perceptual modulation in V1 may be attributable to top-down feedback from MT.

Neural Correlates of Structure-from-Motion Perception in Macaque V1 and MT

Structure-from-motion (SFM) is the perception of three-dimensional shape from motion cues. We used a bistable SFM stimulus, which can be perceived in one of two different ways, to study how neural activity in cortical areas V1 and MT is related to SFM perception. Monkeys performed a depth-order task, where they indicated in which direction the front surface of a rotating SFM cylinder display was moving. To prevent contamination of the neural data because of eye position effects, all experiments with significant effects of radius, vergence, and velocity were excluded. As expected, the activity of ∼50% of neurons in V1 and ∼80% of neurons in MT is affected by the stimulus. Furthermore, the activity of 20% of neurons in area V1 is modulated with the percept. This proportion is higher in MT, where the activity of >60% of neurons is modulated with the percept. In both areas, this perceptual modulation occurs only in neurons with activity that is also affected by the stimulus. The perceptual modulation is not correlated with neural tuning properties in area V1, but it is in area MT. Together, these results suggest that V1 is not directly involved in the generation of the SFM percept, whereas MT is. The perceptual modulation in V1 may be attributable to top-down feedback from MT.