Lee A. Gilroy

Perceptual consequences of centre-surround antagonism in visual motion processing.

Centre–surround receptive field organization is a ubiquitous property in mammalian visual systems, presumably tailored for extracting image features that are differentially distributed over space. In visual motion, this is evident as antagonistic interactions between centre and surround regions of the receptive fields of many direction-selective neurons in visual cortex. In a series of psychophysical experiments we make the counterintuitive observation that increasing the size of a high-contrast moving pattern renders its direction of motion more difficult to perceive and reduces its effectiveness as an adaptation stimulus. We propose that this is a perceptual correlate of centre–surround antagonism, possibly within a population of neurons in the middle temporal visual area. The spatial antagonism of motion signals observed at high contrast gives way to spatial summation as contrast decreases. Evidently, integration of motion signals over space depends crucially on the visibility of those signals, thereby allowing the visual system to register motion information efficiently and adaptively.

Depth of interocular suppression associated with continuous flash suppression, flash suppression, and binocular rivalry.

When conflicting images are presented to the corresponding regions of the two eyes, only one image may be consciously perceived. In binocular rivalry (BR), two images alternate in phenomenal visibility; even a salient image is eventually suppressed by an image of low saliency. Recently, N. Tsuchiya and C. Koch (2005) reported a technique called continuous flash suppression (CFS), extending the suppression duration more than 10-fold. Here, we investigated the depth of this prolonged form of interocular suppression as well as conventional BR and flash suppression (FS) using a probe detection task. Compared to monocular viewing condition, CFS elevated detection thresholds more than 20-fold, whereas BR did so by 3-fold. In subsequent experiments, we dissected CFS into several components. By manipulating the number and timing of flashes with respect to the probe, we found that the stronger suppression in CFS is not due to summation between BR and FS but is caused by the summation of the suppression due to multiple flashes. Our results support the view that CFS is not a stronger version of BR but is due to the accumulated suppressive effects of multiple flashes.

Hearing What the Eyes See Auditory Encoding of Visual Temporal Sequences

When the senses deliver conflicting information, vision dominates spatial processing, and audition dominates temporal processing. We asked whether this sensory specialization results in cross-modal encoding of unisensory input into the task-appropriate modality. Specifically, we investigated whether visually portrayed temporal structure receives automatic, obligatory encoding in the auditory domain. In three experiments, observers judged whether the changes in two successive visual sequences followed the same or different rhythms. We assessed temporal representations by measuring the extent to which both task-irrelevant auditory information and task-irrelevant visual information interfered with rhythm discrimination. Incongruent auditory information significantly disrupted task performance, particularly when presented during encoding; by contrast, varying the nature of the rhythm-depicting visual changes had minimal impact on performance. Evidently, the perceptual system automatically and obligatorily abstracts temporal structure from its visual form and represents this structure using an auditory code, resulting in the experience of “hearing visual rhythms.”

Visual Motion Retards Alternations between Conflicting Perceptual Interpretations

When the visual system is faced with conflicting or ambiguous stimulus information, visual perception fluctuates over time. We found that perceptual alternations are slowed when inducing stimuli move within the visual field, constantly engaging fresh, unadapted neural tissue. During binocular rivalry, dominance durations were longer when rival figures moved compared to when they were stationary, yielding lower alternation rates. Rate was not reduced, however, when observers tracked the moving targets, keeping the images on approximately the same retinal area. Alternations were reliably triggered when rival targets passed through a local region of the visual field preadapted to one of the rival targets. During viewing of a kinetic globe whose direction of rotation was ambiguous, observers experienced fewer alternations in perceived direction when the globe moved around the visual field or when the globes axis of rotation changed continuously. Evidently, local neural adaptation is a key ingredient in the instability of perception.

The Interaction between Binocular Rivalry and Negative Afterimages

Afterimage formation, historically attributed to retinal mechanisms, may also involve postretinal process. Consistent with this notion are results from experiments, reported here, investigating the interaction between binocular rivalry and negative afterimages (AIs). In Experiment 1, one eye was exposed to a grating never consciously experienced by the observer because this grating remained suppressed in rivalry throughout induction (the exclusively dominant stimulus was designed to preclude formation of an AI). As expected, the suppressed grating generated a vivid AI whose orientation could be accurately identified; not surprisingly, the strength of this AI varied with induction contrast. Experiment 2 revealed, however, that the strength of this AI produced during suppression was significantly weaker than the AI produced by that same stimulus when it was visible throughout the entire induction period, implying that some component of AI induction is susceptible to interocular suppression. In Experiment 3, AIs of dichoptic, orthogonally oriented gratings were induced in a way ensuring that one of the two gratings was exclusively dominant during the induction period. Dissimilar monocular AIs engaged in rivalry, as expected, but, surprisingly, the AI induced by the suppressed grating initially dominated. We offer two alternative accounts of this counterintuitive finding, both based on differential neural adaptation.

Physics embedded in visual perception of three-dimensional shape from motion

Visual perception, and by implication underlying neural events, can become unstable when optical information specifying objects is ambiguous. Here we report that one striking form of instability—perceived three-dimensional structure-from-motion (SFM)—can be stabilized when an otherwise ambiguous object appears within a context implying frictional interactions with another rotating object; violations of physical conditions specifying friction disrupt stabilization. Evidently, information about frictional interaction is embedded within neural mechanisms specifying SFM.

A counterchange mechanism for the perception of motion

A computational model for the perception of counterchange-specified motion is examined in detail and compared with various versions of the Reichardt motion detection model [Reichardt, W. (1961). Autocorrelation, a principle for the evaluation of sensory information by the central nervous system. In W. A. Rosenblith (Ed.), Sensory communication (pp. 303-317). New York: Wiley]. The counterchange model is composed of a pair of temporally biphasic subunits at two retinal locations, one detecting decreases and the other increases in input activation. Motion is signaled when both subunits are simultaneously excited, as determined by the multiplicative combination of their transient responses. In contrast with the Reichardt detector, which effectively tracks motion energy and accounts solely for results obtained with standard apparent motion stimuli (a surface is visible at one location, then at another), the counterchange model also accounts for the generalized apparent motion perceived between pairs of simultaneously visible surfaces. This indicates that standard apparent motion can be perceived via the same non-sequential, non-motion-energy mechanism as generalized apparent motion. There is no need for either an explicit delay mechanism to account for optimal motion perception at non-zero inter-stimulus intervals, or for inhibitory interaction between subunits to account for the absence of motion in the detectors null direction (Barlow, H. B., & Levick, W. R., 1965). Both are emergent properties that result from the inhibitory states of the counterchange detectors biphasic subunits. In addition to apparent motion, the counterchange principle potentially accounts for the perception of motion for drifting gratings, the short range motion perceived for random-dot cinematograms, and the motion perceived for continuously moving objects.

Spatial grouping in human vision: Temporal structure trumps temporal synchrony

Temporal information promotes visual grouping of local image features into global spatial form. However, experiments demonstrating time-based grouping typically confound two potential sources of information: temporal synchrony (precise timing of changes) and temporal structure (pattern of changes over time). Here, we show that observers prefer temporal structure for determining perceptual organization. That is, human vision groups elements that change according to the same global pattern, even if the changes themselves are not synchronous. This finding prompts an important, testable prediction concerning the neural mechanisms of binding: patterns of neural spiking over time may be more important than absolute spike synchrony.

Detection of counter-changing contrast: Second-order apparent motion without postrectification motion-energy analysis or salience mapping/feature tracking

The perception of 2nd-order, texture-contrast-defined motion was studied for apparent-motion stimuli composed of a pair of spatially displaced, simultaneously visible checkerboards. It was found that background-relative, counter-changing contrast provided the informational basis for the perception of 2nd-order apparent motion; motion began where contrast changed toward the contrast value of the background checkerboard and ended where contrast changed away from the background value. The perceived apparent motion was not attributable to either postrectification motion-energy analysis or salience-mapping/feature-tracking mechanisms. Parallel results for 1st-order, luminance-defined motion (H. S. Hock, L. A. Gilroy, & G. Harnett, 2002) suggest that counter-changing activation provides a common basis for the perception of both luminance- and texture-contrast-defined apparent motion.

Simultaneity and sequence in the perception of apparent motion

Motion perception usually is accompanied by the phenomenological impression of sequence as objects move through successions of locations. Nonetheless, there is accumulating evidence that sequential information is neither necessary nor sufficient for perceiving motion. It is shown here that apparent motion is specified by counterchange rather than sequence—that is, by co-occurring toward- and away-from-background changes at two spatial locations, regardless of whether the changes are simultaneous or sequential. Motion is perceived from the location of the toward to the location of the away change, even when the changes occur in reverse temporal order. It is not perceived for sequences of away or toward changes, as would be expected if motion were specified by onset or offset asynchronicity. Results previously attributed to onset and offset asynchrony are instead attributable to onsets and offsets occurring in close temporal proximity at the same location. This was consistent with units for detecting away and toward changes that are temporally biphasic; that is, they are excited by changes in one direction and inhibited by immediately preceding or immediately following changes in the opposite direction. These results are accounted for by a model for counterchange-specified motion entailing the biphasic detection of toward and away changes.