We propose a model of the spatial visual processes underlying the identification and representation of the shape of primitive spatial regions. We propose that a region's boundaries are sensed at multiple scales by boundariness detectors that give graded responses, that stimulated boundariness detectors of similar scale, s, connect to one another across a distance that is proportional to their scale, and that they connect via cores, where a core encodes the middles and widths of the region and hence is a trace in (x,y,s), i.e., 3D scale space. 3 INTRODUCTION One of the more impressive feats that the human visual system performs is the identification of individual objects from the continuous distribution of light that falls on the retina. To accomplish this task, the observer uses information from the image to identify regions of interest on the basis of spatial changes in luminance, color, texture, motion, etc. He also interprets information from the image on the basis of prior experi...

Macaque monkeys were presented with continuous rapid serial visual presentation (RSVP) sequences of unrelated naturalistic images at rates of 14-222 msec/image, while neurons that responded selectively to complex patterns (e.g., faces) were recorded in temporal cortex. Stimulus selectivity was preserved for 65% of these neurons even at surprisingly fast presentation rates (14 msec/image or 72 images/sec). Five human subjects were asked to detect or remember images under equivalent conditions. Their performance in both tasks was above chance at all rates (14-111 msec/image). The performance of single neurons was comparable to that of humans and responded in a similar way to changes in presentation rate. The implications for the role of temporal cortex cells in perception are discussed.

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binding across ventral and dorsal processing streams

this memory for recent activity could have many possible biological implementations, as all biochemical processes have a nonzero extent in time. Obvious candidates are the membrane and synaptic mechanisms that have significant time constants. As cortical pyramidal cells may have much longer membrane time constants than previously estimated (possibly 100 ms or more); the simplest possibility is that the membrane could retain an electrical trace of past synaptic activity (Stratford, Mason, Larkman, Major, & Jack, 1989). Alternatively, the trace can be independent of the electrical activation of the neuron. The running average may be kept by a chemical concentration (e.g., calcium; Holmes & Levy, 1990) that is gradually changed by neural activity. Another implementation may involve the activation time constants of synaptic receptor channels (Rhodes, 1992). Nmethyl -D-aspartate (NMDA) receptors, which are believed to be involved in the long-term potentiation of synapses, have relatively slow kinetics. NMDA receptor channels have relatively long opening times, desensitize slowly and incompletely, and can result in large, long-lasting synaptic currents (Lester, Clements, Westbrook, & Jahr, 1990; Mayer, Vyklicky, Benveniste, Patneau, & Williamson, 1991). NMDA currents can therefore last for approximately 200 ms. The neurotransmitter released at an earlier presynaptic activation may still be bound to the receptor at the time of a subsequent postsynaptic depolarization (Holmes & Levy, 1990). Such a mechanism would require the presynaptic activation to precede the postsynaptic activation for potentiation to occur. This condition was in fact required for long-term potentiation in the hippocampus (Levy & Steward, 1983). Because it is the presynaptic activity that is preserved in ...

Form perception from coherent motion is an important aspect of vision. Representations of one-, two- and three-dimensional forms have been found at various stages of cortical processing using random-dot stimuli, whereas representations of biological objects like a walking human being concentrate at higher stages of processing. The perception of biological objects can be induced by sparse dot stimuli that consist of a few dots that mark the joints of the human body [G. Johansson (1973) Percept. Psychophys., 14, 201–211]. In the present study, we aimed to investigate whether neurons in early visual areas that respond to bars and edges defined by luminance contrast also signal bar-like objects from sparse dot stimuli. We studied single neurons with rows of 3–24 dots that were either collinear or scattered within a rectangular form. These dots were moved coherently on a uniform or dotted background, and human observers perceived them as rigid rods or other bar-like objects. We found neurons in the visual cortex of the awake, behaving monkey that responded to these stimuli and were sensitive to the orientation of these objects as for conventional bars or edges. Stimulus conditions that failed to induce these percepts in human observers also evoked weaker responses or none in these neurons. We found these neurons with increasing frequency in areas V1, V2 and V3 ⁄ V3A. The results suggest that the visual cortex not only detects biological objects, but also lines and other bar-like objects from sparse dot stimuli, and that this function evolves at an early stage of processing.

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The human face is a captivating stimulus, even when it is stationary. In motion, however, the face comes to life and offers us a myriad of information about the intent and personality of its owner. Through facial movements such as expressions, we can gauge a person’s current state of mind. By perceiving the movements of the mouth as a friend speaks, a conversation becomes more intelligible in a noisy environment. Through the rigid movement of the head and the direction of eye gaze, we can follow another person’s focus of attention in a crowded room. The amount and diversity of social information that can be conveyed by a face assures its place as a central focal object in any scene. Beyond the rich communication signals that we perceive in facial expressions, head orientation, eye gaze, and facial speech motions, it is also pertinent to ask whether the movements of a face help us to remember a person. The answer to this question can potentially advance our understanding of how the complex tasks we perform with faces, including those having to do with social interaction and memory, co-exist in a neural processing network. It can also shed light on

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www.elsevier.com/locate/visres Invariance of long-term visual priming to scale, reflection, translation, and hemisphere