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T D Albright

Publications and source records attributed to T D Albright.

17 recordsLinked to original sources
PubMed1992

Neural correlates of perceptual motion coherence.

The motions of overlapping contours in a visual scene may arise from the physical motion(s) of either a single or multiple surface(s). A central problem facing the visual motion system is that of assigning the most likely interpretation. The rules underlying this perceptual decision can be explored using a visual stimulus formed by superimposing two moving gratings. The resultant percept is either that of a single coherently moving 'plaid pattern' (coherent motion) or of the two component gratings sliding noncoherently across one another (noncoherent motion). When plaid patterns are configured to mimic one transparent grating overlying another, the percept of noncoherent motion dominates. We now report that neurons in the visual cortex of rhesus monkeys exhibit changes in direction tuning that parallel this perceptual phenomenon: sensitivity to the motions of the component gratings is enhanced under conditions that favour the perception of noncoherent motion. These results challenge models of cortical visual processing that fail to take into account the contribution of figural image segmentation cues to the analysis of visual motion.

Animals↗
PubMed1992

Form-cue invariant motion processing in primate visual cortex.

The direction and rate at which an object moves are normally not correlated with the manifold physical cues (for example, brightness and texture) that enable it to be seen. As befits its goals, human perception of visual motion largely evades this diversity of cues for image form; direction and rate of motion are perceived (with few exceptions) in a fashion that does not depend on the physical characteristics of the object. The middle temporal visual area of the primate cerebral cortex contains many neurons that respond selectively to motion in a particular direction and is an integral part of the neural substrate for perception of motion. When stimulated with moving patterns characterized by one of three very diverse cues for form, many middle temporal neurons exhibited similar directional tuning. This lack of sensitivity for figural cue characteristics may allow the uniform perception of motion of objects having a broad spectrum of physical cues.

Animals↗
PubMed1992

Heterochromatic Fusion Nystagmus: its use in estimating chromatic equiluminance in humans and monkeys.

The use of chromatic patterns that are equated for luminance has become increasingly popular in psychophysical and neurophysiological studies of visual processing. The currently available techniques for equating different colors for brightness rely upon human reports of perceptual events that are reduced at some luminance ratio. We report here the results of a study using a technique we have recently developed that produces a vivid and compelling motion percept only at isoluminance. That is, unlike previous methods, this technique relies upon a perceptual event (motion) that actually becomes more salient at isoluminance. We also observed that the optokinesis generated by the moving pattern mirrors the perceptual reports at all luminance ratios. If used in this manner, the technique can provide an estimate of chromatic isoluminance in a variety of species and can be used to corroborate a human subject's perceptual experience.

Animals↗
PubMed1992

Motion coherency rules are form-cue invariant.

Object features can be made manifest by differences in a variety of physical attributes or figural cues. In this study we provide evidence that motion signals arising from different figural cues can be combined to produce a percept of a coherently moving (heterogeneous-cue) pattern. Moreover, as is true for homogeneous-cue patterns, similarity along the dimensions of contrast (we introduce the idea of contrast equivalence) and spatial frequency determines whether coherent motion is perceived. These results are in accordance with recent physiological evidence demonstrating that directional selectivity of many neurons in cortical visual area MT is invariant over changes in the figural cue that defines the moving stimulus ("form-cue invariance").

Contrast Sensitivity↗
PubMed1990

Transparency and coherence in human motion perception.

When confronted with moving images, the visual system often must decide whether the motion signals arise from a single object or from multiple objects. A special case of this problem arises when two independently moving gratings are superimposed. The gratings tend to cohere and move unambiguously in a single direction (pattern motion) instead of moving independently (component motion). Here we report that the tendency to see pattern motion depends very strongly on the luminance of the intersections (that is, to regions where the gratings overlap) relative to that of the gratings in a way that closely parallels the physics of transparency. When the luminance of these regions is chosen appropriately, pattern motion is destroyed and replaced by the appearance of two transparent gratings moving independently. The observations imply that motion detecting mechanisms in the visual system must have access to tacit 'knowledge' of the physics of transparency and that this knowledge can be used to segment the scene into different objects. The same knowledge could, in principle, be used to avoid confusing shadows with real object boundaries.

Humans↗
PubMed1990

A new technique for estimating chromatic isoluminance in humans and monkeys.

Current approaches to the problem of equating different colors for luminance (chromatic isoluminance) rely upon human reports of perceptual events that are reduced at some luminance ratio. In this report, a technique is described that evokes a vivid percept of motion of a textured pattern only at isoluminance. Furthermore, in both humans and monkeys, the moving stimulus produces a striking optokinetic response in the same direction as the perceived motion. If used in this manner, the technique can provide an estimate of chromatic isoluminance in a variety of species and be used to corroborate a human subject's perceptual judgement.

Animals↗
PubMed1990

Afferent basis of visual response properties in area MT of the macaque. II. Effects of superior colliculus removal.

In a previous study (Rodman et al., 1989), we found that many neurons in the middle temporal area (MT) of the macaque monkey remain visually responsive and directionally selective after striate cortex lesions or cooling. In the present study, we examined the effects of superior colliculus (SC) lesions and combined lesions of striate cortex and the SC on the visual properties of MT neurons. Removal of the SC alone had no effect on the proportion of visually responsive cells, strength of direction selectivity and direction tuning, orientation tuning, receptive field size, or binocularity in MT. There was, however, a slight increase in response strength to both stationary and moving slit stimuli. In contrast to the minor effects of SC lesions alone, addition of an SC lesion to striate cortex damage abolished all visual responsiveness in area MT. The results indicate that pathways damaged by the SC lesion are not necessary for most of the properties of MT neurons found in the intact animal, although these pathways are capable of sustaining considerable visual responsiveness and direction selectivity when striate input is removed.

Animals↗
PubMed1989

Single-unit analysis of pattern-motion selective properties in the middle temporal visual area (MT).

The middle temporal visual area (MT) in macaque extrastriate cortex is characterized by a high proportion of neurons selective for the direction of stimulus motion, and is thus thought to play an important role in motion perception. Previous studies identified a population of cells in MT that appeared capable of coding the motion of whole visual patterns independent of the motions of contours within them (Gizzi et al. 1983; Movshon et al. 1985). These "pattern-motion selective" neurons are unlike motion sensitive cells that have been observed at earlier stages of the visual system. Using very different criteria, we have also previously identified an apparently functionally distinct group of MT neurons (Albright 1984). We predicted that these "Type II" neurons correspond to the pattern-motion neurons. In the present study, we have applied both sets of criteria to individual neurons in MT and found that these two differently defined sets of cells actually form the same population. These results support the idea that MT contributes to a specialized type of motion processing which reflects the integrity of normal perception.

Animals↗
PubMed1989

Centrifugal directional bias in the middle temporal visual area (MT) of the macaque.

We have examined the distribution of preferred directions of motion for neurons in the middle temporal visual area (MT) of the macaque. We found a marked anisotropy favoring directions that are oriented away from the center of gaze. This anisotropy is present only among neurons with peripherally located receptive fields. This peripheral centrifugal directionality bias corresponds well to the biased distribution of motions characteristic of optic flow fields, which are generated by displacement of the visual world during forward locomotion. The bias may facilitate the processing of this common form of visual stimulation and could underlie previously observed perceptual anisotropies favoring centrifugal motion. We suggest that the bias could arise from exposure of modifiable cortical circuitry to a naturally occurring form of selective visual experience.

Animals↗
PubMed1989

Afferent basis of visual response properties in area MT of the macaque. I. Effects of striate cortex removal.

The middle temporal area (MT) of the macaque monkey is a region of extrastriate cortex involved in the analysis of visual motion. MT receives strong projections from striate cortex and from area V2, which is dependent on striate for visual responsiveness. Accordingly, the visual properties of MT neurons have been thought to reflect the further processing of its input from striate cortex. We examined the dependence of MT activity on pathways deriving from striate cortex by recording from MT neurons following removal of their striate input. Repeated recordings in area MT were made in 4 hemispheres of anesthetized macaques following either partial or total ablations of striate cortex. Cells in MT were tested for responsiveness, selectivity for direction of motion and direction tuning, and ocular dominance. Receptive fields were also plotted. In an additional animal, we recorded from MT neurons during reversible cooling of the central representation in striate cortex. We found that striate cortex removal or inactivation did not abolish the visual responsiveness of the majority of MT cells. Although the residual responses were generally much weaker than in the intact animal, direction selectivity and binocularity were still present. Moreover, receptive field size and overall topography appeared unaltered.

Afferent Pathways↗
PubMed1987

Local precision of visuotopic organization in the middle temporal area (MT) of the macaque.

The representation of the visual field in the middle temporal area (MT) was examined by recording from single neurons in anesthetized, immobilized macaques. Measurements of receptive field size, variability of receptive field position (scatter) and magnification factor were obtained within the representation of the central 25 degree. Over at least short distances (less than 3 mm), the visual field representation in MT is surprisingly orderly. Receptive field size increases as a linear function of eccentricity and is about ten times larger than in V1 at all eccentricities. Scatter in receptive field position at any point in the visual field representation is equal to about one-third of the receptive field size at that location, the same relationship that has been found in V1. Magnification factor in MT is only about one-fifth that reported in V1 within the central 5 degree but appears to decline somewhat less steeply than in V1 with increasing eccentricity. Because the smaller magnification factor in MT relative to V1 is complemented by larger receptive field size and scatter, the point-image size (the diameter of the region of cortex activated by a single point in the visual field) is roughly comparable in the two areas. On the basis of these results, as well as on our previous finding that 180 degrees of axis of stimulus motion in MT are represented in about the same amount of tissue as 180 degrees of stimulus orientation in V1, we suggest that a stimulus at one point in the visual field activates at least as many functional "modules" in MT as in V1.

Animals↗
PubMed1987

Coding of visual stimulus velocity in area MT of the macaque.

We have studied the interaction of the direction and speed selectivities of neurons in cortical visual area MT of the macaque monkey. For a given cell, preferred direction and the shape of the direction tuning curve for moving edges were similar at different stimulus speeds, and deviations from the optimal speed did not systematically alter direction tuning bandwidth. Similar speed tuning was obtained for responses to motion in the preferred and anti-preferred directions even when the response to anti-preferred motion was an inhibitory one. The results are discussed in terms of the unique contributions of area MT to visual motion analysis.

Action Potentials↗
PubMed1984

Columnar organization of directionally selective cells in visual area MT of the macaque.

We recorded from single neurons in visual area MT of the macaque in order to examine the spatial distribution of its directionally selective cells. The animals were paralyzed and anesthetized with nitrous oxide. All MT neurons (n = 614) responded better to moving stimuli than to stationary stimuli. For 55% of the neurons, responses to moving stimuli were independent of stimulus color, shape, length, or orientation. For the remaining cells, stimulus length affected the response magnitude and tuning bandwidth but not the preferred direction. MT neurons were divided into four categories on the basis of their sensitivity to moving stimuli: 60% responded exclusively to one direction of motion, 24% responded best to one direction with a weaker response in the opposite direction, 8% responded equally well to two opposite directions of motion, and 8% responded equally well to all directions of motion. The direction preferences of successively sampled cells on a penetration either changed by small increments or occasionally by approximately 180 degrees. Thus, there is a systematic representation of direction of motion. The representation of axis of motion, i.e., the orientation of the path along which a stimulus moves, is more continuous than the representation of direction of motion. There was a systematic relationship between penetration angle and rate of change of preferred axis of motion, indicating that cells with a similar axis of motion preference are arranged in vertical columns. Furthermore, axis of motion columns appear to exist in the form of continuous slabs in area MT. The size of these slabs is such that 180 degrees of axis of motion are represented in 400-500 micron of cortex. There was also a systematic relationship between penetration angle and frequency of 180 degrees reversals, indicating that cells with a similar direction of motion preference are also organized in vertical columns and cells with opposite direction preferences are located in adjacent columns within a single axis of motion column. Just as in macaque striate cortex where approximately 500 micron of cortex contain the mechanism for the local analysis of stimulus orientation, so in MT approximately 500 micron of cortex contain the mechanism for the local analysis of stimulus motion.

Animals↗
PubMed1984

Direction and orientation selectivity of neurons in visual area MT of the macaque.

We recorded from single neurons in the middle temporal visual area (MT) of the macaque monkey and studied their direction and orientation selectivity. We also recorded from single striate cortex (V1) neurons in order to make direct comparisons with our observations in area MT. All animals were immobilized and anesthetized with nitrous oxide. Direction selectivity of 110 MT neurons was studied with three types of moving stimuli: slits, single spots, and random-dot fields. All of the MT neurons were found to be directionally selective using one or more of these stimuli. MT neurons exhibited a broad range of direction-tuning bandwidths to all stimuli (minimum = 32 degrees, maximum = 186 degrees, mean = 95 degrees). On average, responses were strongly unidirectional and of similar magnitude for all three stimulus types. Orientation selectivity of 89 MT neurons was studied with stationary flashed slits. Eighty-three percent were found to be orientation selective. Overall, orientation-tuning bandwidths were significantly narrower (mean = 64 degrees) than direction-tuning bandwidths for moving stimuli. Moreover, responses to stationary-oriented stimuli were generally smaller than those to moving stimuli. Direction selectivity of 55 V1 neurons was studied with moving slits; orientation selectivity of 52 V1 neurons was studied with stationary flashed slits. In V1, compared with MT, direction-tuning bandwidths were narrower (mean = 68 degrees). Moreover, V1 responses to moving stimuli were weaker, and bidirectional tuning was more common. The mean orientation-tuning bandwidth in V1 was also significantly narrower than that in MT (mean = 52 degrees), but the responses to stationary-oriented stimuli were of similar magnitude in the two areas. We examined the relationship between optimal direction and optimal orientation for MT neurons and found that 61% had an orientation preference nearly perpendicular to the preferred direction of motion, as is the case for all V1 neurons. However, another 29% of MT neurons had an orientation preference roughly parallel to the preferred direction. These observations, when considered together with recent reports claiming sensitivity of some MT neurons to moving visual patterns (39), suggest specific neural mechanisms underlying pattern-motion sensitivity in area MT. These results support the notion that area MT represents a further specialization over area V1 for stimulus motion processing. Furthermore, the marked similarities between direction and orientation tuning in area MT in macaque and owl monkey support the suggestion that these areas are homologues.

Animals↗
PubMed1984

Stimulus-selective properties of inferior temporal neurons in the macaque.

Previous studies have reported that some neurons in the inferior temporal (IT) cortex respond selectively to highly specific complex objects. In the present study, we conducted the first systematic survey of the responses of IT neurons to both simple stimuli, such as edges and bars, and highly complex stimuli, such as models of flowers, snakes, hands, and faces. If a neuron responded to any of these stimuli, we attempted to isolate the critical stimulus features underlying the response. We found that many of the responsive neurons responded well to virtually every stimulus tested. The remaining, stimulus-selective cells were often selective along the dimensions of shape, color, or texture of a stimulus, and this selectivity was maintained throughout a large receptive field. Although most IT neurons do not appear to be "detectors" for complex objects, we did find a separate population of cells that responded selectively to faces. The responses of these cells were dependent on the configuration of specific face features, and their selectivity was maintained over changes in stimulus size and position. A particularly high incidence of such cells was found deep in the superior temporal sulcus. These results indicate that there may be specialized mechanisms for the analysis of faces in IT cortex.

Animals↗
PubMed1983

Shape recognition and inferior temporal neurons.

Inferior temporal cortex plays an important role in shape recognition. To study the shape selectivity of single inferior temporal neurons, we recorded their responses to a set of shapes systematically varying in boundary curvature. Many inferior temporal neurons were selective for stimuli of specific boundary curvature and maintained this selectivity over changes in stimulus size or position. The method of describing boundary curvature was that of Fourier descriptors.

Animals↗