Orientation maps of subjective contours in visual cortex.

Responses to subjective contours in visual cortical areas V1 and V2 in adult cats were investigated by optical imaging of intrinsic signals and single-unit recording. Both V1 and V2 contain maps of the orientation of subjective gratings that have their basis in specific kinds of neuronal responses to subjective orientations. A greater proportion of neurons in V2 than in V1 show a robust response to subjective edges. Through the use of subjective stimuli in which the orientation of the luminance component is invariant, an unmasked V1 response to subjective edges alone can be demonstrated. The data indicate that the processing of subjective contours begins as early as V1 and continues progressively in higher cortical areas.

Changing objects lead briefly flashed ones.

Continuous, predictable events and spontaneous events may coincide in the visual environment. For a continuously moving object, the brain compensates for delays in transmission between a retinal event and neural responses in higher visual areas. Here we show that it similarly compensated for other smoothly changing features. A disk was flashed briefly during the presentation of another disk of continuously changing color, and observers compared the colors of the disks at the moment of flash. We also tested luminance, spatial frequency and pattern entropy; for all features, the continuously changing item led the flashed item in feature space. Thus the visual system's ability to compensate for delays in information about a continuously changing stimulus may extend to all features. We propose a model based on backward masking and priming to explain the phenomenon.

Compression of space in visual memory.

Human observers had to point to the location of a briefly presented target by means of a mouse after a brief delay following target offset. It was found that observers systematically mislocalized the target closer to the center of gaze, and to visually salient markers in the visual display. A perceptual judgment task revealed that these errors in localization were independent of whether or not eye movements were made, and even of planning for them, thereby demonstrating that the effect was a perceptual phenomenon, not a sensorimotor one. Further experiments demonstrated clearly that the magnitude of the time interval between target presentation and judgment regarding its spatial location was the critical parameter. A longer time interval between the event and its report enhanced significantly the amplitude of compression, thus establishing this phenomenon as a visual memory effect. We conclude that visual memory of spatial location is distorted over time in a systematic, monotonic fashion as a result of the sustained fixation of the observer on a fixed location during and shortly after target presentation, or by the continual presence of stable, salient landmarks in the environment.

In space, the past can be recast but not the present.

We address the relationship between perception and spatial, working memory. Specifically, we argue that perceptual experience following the creation of a representation of target location affects it in a systematic way. We designed a motor task in which observers had to point to the initial or final position of a horizontally drifting target embedded in a vertically drifting background. The target was perceived as having an illusory motion component in a direction opposite that of the inducer dots [Duncker, 1938, Source Book of Gestalt Psychology (London: Kegan Paul, Trench, Trubner and Co)]. For both positions, there was an identical time delay before the observer could respond. Nonetheless, estimates of the initial target position were significantly biased by the illusion in a direction opposite the perceived target motion, and both bias and variability were significantly greater than those of the target's final position. In prior studies on positional accuracy with induced displacement, a delay before a pointing response led to an unbiased position estimate obtained without delay to become biased, leading investigators to argue for a long-lasting, inaccurate cognitive system that overrules an accurate, nonetheless transient, motor one (Bridgeman et al, 1997, Perceptual Psychology 59 456-469). Since the same motor task with identical delay on either position yielded different outcomes, a hypothesis based on distinct motor and cognitive representations of visual space is untenable here. Instead, we argue that an online representation of the target's original position is updated in an ongoing fashion in order to reconcile the perceived illusion with the veridically perceived present (current target location).

Temporal modulation of spatial borders in rat barrel cortex.

We examined the effects of varying vibrissa stimulation frequency on intrinsic signal and neuronal responses in rat barrel cortex. Optical imaging of intrinsic signals demonstrated that the region of cortex activated by deflection of a single vibrissa at 1 Hz is more diffuse and more widespread than the territory activated at 5 or 10 Hz. With the use of two different paradigms, constant time of stimulation and constant number of vibrissa deflections, we showed that the optically imaged spread of activity is more discrete at higher stimulation frequencies. We combined optical imaging with multiple electrode recording and confirmed that the neuronal response to individual vibrissa stimulation at the optically imaged center of activity is greater than the response away from the imaged center. Consistent with the imaging data, these recordings also showed no response to a second vibrissa deflection at 5 Hz at a peripheral recording site, though there was a significant response to a second vibrissa deflection at 1 Hz at the same peripheral site. These findings demonstrate that vibrissa stimulation at higher frequencies leads to more focused physiological responses in cortex. Thus the spread of activation in rat barrel cortex is modulated in a dynamic fashion by the frequency of vibrissa stimulation.