Incremental grouping of image elements in vision

One important task for the visual system is to group image elements that belong to an object and to segregate them from other objects and the background. We here present an incremental grouping theory (IGT) that addresses the role of object-based attention in perceptual grouping at a psychological level and, at the same time, outlines the mechanisms for grouping at the neurophysiological level. The IGT proposes that there are two processes for perceptual grouping. The first process is base grouping and relies on neurons that are tuned to feature conjunctions. Base grouping is fast and occurs in parallel across the visual scene, but not all possible feature conjunctions can be coded as base groupings. If there are no neurons tuned to the relevant feature conjunctions, a second process called incremental grouping comes into play. Incremental grouping is a time-consuming and capacity-limited process that requires the gradual spread of enhanced neuronal activity across the representation of an object in the visual cortex. The spread of enhanced neuronal activity corresponds to the labeling of image elements with object-based attention

Cue-invariant shape selectivity of macaque inferior temporal neurons

The perception of shape is independent of the size and position of the shape and also of the visual cue that defines it. The same shape can be recognized whether defined by a difference in luminance, by motion, or by texture. Experiments showed that the shape selectivity of individual cells in the macaque inferior temporal cortex did not vary with the size and position of a shape and also did not vary with the visual cue used to define the shape. This cue invariance was true for static luminance and texture cues as well as for relative motion cues-that is, for cues that are processed in ventral and dorsal visual pathways. The properties of these inferior temporal cells meet the demands of cue-invariant shape coding

Cerebral regions processing first- and higher-order motion in an opposed-direction discrimination task

Using PET, we studied the processing of different types of motion in an opposed-direction discrimination task. We used first-order motion and two types of higher-order motion (presented as moving gratings with stripes defined by flickering texture and kinetic boundaries, respectively). In these experiments, we found that all types of motion activate a common set of cortical regions when comparing a direction discrimination task to a detection of the dimming of the fixation point. This set includes left hV3A, bilateral hMT/ V5þ and regions in the middle occipital gyrus, bilateral activations in the posterior and anterior parts of the intraparietal sulcus, bilateral precentral gyrus, medial frontal cortex and regions in the cerebellum. No significant differences were observed between different types of motion, even at low statistical thresholds. From this we conclude that, under our experimental conditions, the same cerebral regions are involved in the processing of first-order and higher-order motion in an opposed-direction discrimination task

Vasoactive intestinal polypeptide promotes sleep without effects on brain temperature in rats at night

The possible sleep-promoting activity of vasoactive intestinal polypeptide (VIP) was contrasted with the physiological sleep suppression in the diurnal active period through the i.c.v, injection of 100 ng VIP into rats at dark onset. The sleep-wake activity and brain temperature (Tbr) were recorded for 24 h (dark period and light period, 12 h each), and the effects were evaluated with respect to records obtained after artificial cerbrospinal fluid injection. Without altering the normal course of Tbr, VIP induced a prompt and persistent increase of sleep. Wakefulness was significantly suppressed and non-REM sleep increased for 6 h, while REM sleep increased for 3 h. The obvious sleep-promoting action of VIP, unrelated to thermoregutatory effects, supports the notion that the peptide might be involved in sleep regulation

Effects of Surface Cues on Macaque Inferior Temporal Cortical Responses

Humans are able to recognize objects when surface details, such as colour, texture and luminance gradients, are not available. By systematically eliminating colour, texture, shading, contrast and inner contours from given objects, we tested whether certain shape-selective inferior temporal cortex (IT) neurons of awake rhesus monkeys remain selective for these objects as the surface information is reduced. In psychophysical experiments, we established that the rhesus monkey can identify the shape of a coloured object largely independently of its surface characteristics and, to a lesser degree, of its inner contours. Shape selectivity of the neurons does not change when texture and shading are concealed. The responsiveness of the neurons is also affected by the removal of these surface attributes. The IT neurons were found to respond highly similarly to objects brighter or darker than their background. Selectivity for shape is preserved when the contrast is reversed. Deletion of the inner contours, outlining the main parts of the objects, did not affect the responses and selectivity of the IT neurons. These findings indicate that the IT can contribute to the invariant perception of objects having different surface details