Dynamics of subjective contour formation in the early visual cortex

Abstract

To elucidate the roles of visual areas V1 and V2 and their interaction in early perceptual processing, we studied the responses of V1 and V2 neurons to statically displayed Kanizsa figures. We found evidence that V1 neurons respond to illusory contours of the Kanizsa figures. The illusory contour signals in V1 are weaker than in V2, but are significant, particularly in the superficial layers. The population averaged response to illusory contours emerged 100 msec after stimulus onset in the superficial layers of V1, and around 120 -190 msec in the deep layers. The illusory contour response in V2 began earlier, occurring at 70 msec in the superficial layers and at 95 msec in the deep layers. The temporal sequence of the events suggests that the computation of illusory contours involves intercortical interaction, and that early perceptual organization is likely to be an interactive process. W hen viewing the Kanizsa display shown in Fig. 1a, we perceive the borders of a square even in regions of the image where there is no direct visual evidence for them. This is one example of the phenomenon of illusory or subjective contours (1). This perceptual phenomenon has been reported by von der Heydt and colleagues (2, 3) to possess a direct physiological correlate in macaque area V2, where some neurons were found to respond to an illusory contour moving across their receptive fields. In contrast, they failed to observe responses to illusory contours in area V1. The apparent absence of illusory-contour responses in area V1 is puzzling both because there are recurrent pathways from V2 to V1 and because interaction between modules is a key feature of many models for early perceptual organization (4-7). Moreover, other groups have shown that neurons in area V1 do detect subjective contours defined indirectly in other ways, for example by the fracture line where lines or out of phase sine wave gratings abut (8, 9). Because of the nature of their stimuli, these studies (8, 9) did not resolve the question of whether their results would apply to the illusory contours as studied by Von der Heydt and colleagues. In light of these considerations, we decided to reexamine the issue of neural responses to illusory contours in areas V1 and V2. By using a technique designed to call attention to the illusory square and employing a static display that allowed tracking the temporal evolution of responses, we have found that neurons in area V1 do respond to illusory contours, although at a latency greater than that in V2.We conducted the following neurophysiological experiment on two awake behaving rhesus monkeys. In each trial, while the monkey was fixating a red dot on the screen within a 0.5°fixation window, a sequence of four stimuli was presented. The presentation of each stimulus in the sequence lasted for 400 msec. On completion of the sequence, the monkey had to make a saccadic eye movement to another red dot that appeared at a random position on the screen to complete the trial. The set of test stimuli included a Kanizsa figure with...

The Role of Dendritic Signaling in the Anesthetic Suppression of Consciousness

“…82,88 Along the same lines, latency data from single-unit recordings in monkeys suggest that the perception of illusory contours (where, again, subjective experience diverges from the physical nature of the stimulus) is signaled from higher-order to lower-order visual cortices. 77 Similar findings exist for the somatosensory modality: the subjective intensity of tactile stimuli is reflected in top-down signals that reach layer 1 of the primary somatosensory cortex from higher-level areas rather than in the thalamocortical signals that initially arrive in layer 4. 75,76 As an additional observation, which applies equally to the visual, auditory, and somatosensory modalities, there is an interdependency of the latency of sensory cortex responses and their correlation with conscious experience: although early activity in the sensory areas appears to be strictly stimulus bound, later activity, which reflects top-down signals from higher-order cortices, is correlated more closely with the subject's conscious percept.…”

“…Importantly, there is substantial evidence to suggest that top-down processes not only play an important role in sensory processing in general but also are crucial for conscious perception, in particular [75][76][77][78][79][80][81][82] (see also the review articles by Pollen, 83 Lamme and Roelfsema, 84 Bullier, 85 Hochstein and Ahissar, 86 and Meyer 87 ). For example, when subjects perceive apparent motion (as is the case when two dots in different locations are seen in rapid alteration, creating the impression that a single dot is moving from one location to the other), there is V1 activity along the apparent motion trace (where there is no actual visual stimulus), and this activity is induced by top-down signals.…”

Illusory contours: a window onto the neurophysiology of constructing perception

“…The IC effect is indistinguishable both when the illusory shape is completed modally as well as amodally [36]. This latter finding provides another argument against a mechanism mediated solely by feed-forward activity in V2/V1, as single-unit recordings either failed to observe sensitivity in V2 to amodal completion [16,43] …”

Are there interactive processes in speech perception?

“…Although interactive processing remains a controversial proposal in neuroscience as well as psychology, the interactive approach is supported by several findings from neuroscience. We mention three that we think are particularly telling: (i) Bi-directional connections are the rule rather than the exception in connections between areas in the brain [10]; (ii) Inactivating 'downstream' motion sensitive cortex (area MT) reduces sensitivity to motion in 'upstream' visual areas (V1 and V2), as would be expected if MT integrates motion information across the visual field and feeds it back to lower areas [11]; (iii) Illusory contours in Kanizsa figures activate edge-sensitive neurons in low-level visual areas V1 and V2 [12]. This last effect is delayed relative to the direct bottom-up activation of these neurons that occurs for real edges, as though it were mediated by interactive processes distributed across several visual areas [12].…”