A sustained input to the direction-selective mechanism in cat striate cortex neurons

Transient—Sustained Input to Directionally Selective Motion Mechanisms

The characteristics of the sustained input to directionally selective motion sensors were examined in three human psychophysical studies on directional-motion discrimination. Apparent motion was produced by displaying a group of dots in two frames (F1 and F2), where F2 was a translated version of F1. All stimuli included parts that contained both F1 and F2 (combined images) and parts containing only F1 or F2 (single images). All displays began with a single image (F1), continued with the combined image, and ended with F2. Six durations of single and of combined images (10, 20, 40, 80, 160, or 320 ms) were crossed factorially. As the duration of the single image was increased, perception of directional motion first improved, and then declined at longer durations. This outcome contrasted with the monotonic increment obtained in earlier studies under low-luminance conditions. To account for the entire pattern of results, earlier models of the Reichardt motion sensor were modified so as to include a mixed transient – sustained input to one of the filters of the sensor. Predictions from the new model were tested and confirmed in two experiments carried out under both low-luminance and high-luminance viewing conditions.

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 detector's null direction (Barlow, H. B., & Levick, W. R., 1965). Both are emergent properties that result from the inhibitory states of the counterchange detector's 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.

Processing of second-order stimuli in the visual cortex

Naturally occurring visual stimuli are rich in examples of objects delineated from their backgrounds simply by differences in luminance, so-called first-order stimuli, as well as those defined by differences of contrast or texture, referred to as second-order stimuli. Here we provide a brief overview of visual cortical processing of second-order stimuli, as well as some comparative background on first-order processing, concentrating on single-unit neurophysiology, but also discussing relationships to human psychophysics and to neuroimaging. The selectivity of visual cortical neurons to orientation, spatial frequency, and direction of movement of first-order, luminance-defined stimuli is conventionally understood in terms of simple linear filter models, albeit with some minor nonlinearities such as thresholding and gain control. However, these kinds of models fail entirely to account for responses of neurons to second-order stimuli such as contrast envelopes, illusory contours, or texture borders. Second-order stimuli constructed from sinusoidal components have been used to analyze the neurophysiological mechanisms of such responses; these experiments demonstrate that the same neuron can exhibit three distinct kinds of tuning to spatial frequency, and also to orientation. These results can be understood in terms of a type of nonlinear 'filter-->rectify-->filter' model, which has been widely used in human psychophysics. Finally, several general issues will be discussed, including potential artifacts in experiments with second-order stimuli, and strategies for avoiding or controlling for them; caveats about definitions of first- vs. second-order mechanisms and stimuli; the concept of form-cue invariance; and the functional significance of second-order processing.

A spatial gradient of acceleration and temporal extension underlies three illusions of motion

If an object (or cue), is presented and shortly afterwards a line is drawn with one end near to the object, motion away from the object location is induced within the line. This line-motion illusion has best been explained by postulating a facilitative spatial gradient that accelerates signal transmission most strongly near to the object, and less so with increasing distance away from the object. This simple accelerative-gradient model was tested in four experiments by either briefly presenting the line, or replacing the line rendering with a dot moving at high velocity towards (or away from) the initial object location. Observers first perceived motion away from the cue followed by motion towards the cue (hence this new illusion is referred to as the "two motion percepts", or TMP illusion). The generality of the TMP illusion was investigated through the reports of forty-five inexperienced undergraduates who were presented with TMP displays. Observers who were asked to pictorially reproduce their motion experience, drew a line expanding away from the cue then contracting back towards it 85% of the time. Over 90% of individuals reported experiencing the illusion with a quickly moving dot. The effects of several presentation parameters were investigated by the moving-dot method, and it was concluded that the accelerative-gradient model by itself was inadequate to explain TMP phenomena. Two extended versions of the gradient model are proposed that place the locus of the TMP effects in properties of motion detection mechanisms or in temporal aspects of visual-signal transmission.

Psychophysical evidence of a sustained input to directionally selective motion mechanisms

Human psychophysical evidence congruent with neurophysiological findings of a sustained input to directionally selective motion sensors in cat visual cortex is reported. Apparent motion was produced by displaying a group of dots in two frames (F1 and F2), where F2 was a translated version of F1. All stimulus sequences included a period during which F1 and F2 were displayed concurrently (combined images) and a period during which only F1 or F2 was on display (single images). There were three stimulus sequences: a display beginning with combined and ending with single image, a display beginning with single and ending with combined image, and a display beginning with F1, continuing with combined image, and ending with F2. Six durations of single and of combined images (10, 20, 40, 80, 160, and 320 ms) were crossed factorially in each stimulus sequence. Directional motion was seen easily at long durations of the single image in all stimulus sequences, as would be expected on the basis of a sustained input to the directional-motion-sensing mechanisms. Perception of directional motion improved with the duration of single images, but declined as the duration of combined images was increased. Baker and Cynader's model could account for the effect of duration of single images, but not for the effect of duration of combined images. An elaborated version of the model provides a good qualitative match to all empirical findings.