Role of posterior parietal cortex in the recalibration of visually guided reaching

Visually guided reaching requires complex neural transformations to link visual and proprioceptive inputs with appropriate motor outputs. Despite the complexity of these transformations, hand-eye coordination in humans is remarkably flexible, as demonstrated by the ease with which reaching can be adapted to distortions in visual feedback. If subjects attempt to reach to visual targets while wearing displacing prisms, they initially misreach in the direction of visual displacement. Given feedback about their reaching errors, however, they quickly adapt to the visual distortion. This is shown by the gradual resumption of accurate reaching while the prisms remain in place, and by the immediate onset of reaching errors in the opposite direction after the prisms have been removed. Despite an abundance of psychophysical data on adaptation to prisms, the functional localization of this form of sensorimotor adaptation is uncertain. Here we use positron emission tomography (PET) to localize changes in regional cerebral blood flow (rCBF) in subjects who performed a prism-adaptation task as well as a task that controlled for the sensory, motor and cognitive conditions of the adaptation experiment. Difference images that reflected the net effects of the adaptation process showed selective activation of posterior parietal cortex contralateral to the reaching limb.

The orientation specificity of two visual after-effects

1. Inspection of a high-contrast adapting grating produces two visual after-effects: (a) the contrast threshold is raised for test gratings of similar spatial frequency to that of the adapting pattern and (b) the apparent spatial frequency of test gratings shifts away from that of the adapting grating-higher frequencies seem higher and lower ones lower than they really are.2. Both after-effects are orientation-specific. A horizontal adapting grating influences neither the threshold nor the apparent spatial frequency of vertical test gratings.3. The magnitude of the two after-effects was measured with vertical test gratings as a function of (a) tilt of a high-contrast adapting grating and (b) contrast of a vertical adapting grating.4. At all frequencies of the test grating, the decline of both after-effects produced by an increase in tilt of approximately 6(3/4) degrees could be matched by a reduction in contrast by a factor of 2.5. We take this as evidence for a common neural origin for these two visual phenomena.

The perceived spatial frequency shift: evidence for frequency-selective neurones in the human brain

1. Prolonged observation of a high-contrast grating pattern causes an apparent shift in the spatial frequency of gratings subsequently viewed with the same retinal region. Gratings of higher and lower frequency than the adapting pattern seem, respectively, higher and lower than in fact they are.2. There is no significant after-effect at the adapting frequency itself nor at frequencies more than two octaves away.3. For very low adapting frequencies, the after-effect remains centred at about 3.0 c/deg and declines in strength as the adapting frequency is successively lowered.4. The magnitude of the after-effect increases with the contrast of the adapting grating and the length of time spent in adaptation. It takes several hours to recover completely from 30 min adaptation.5. The phenomenon is orientation-specific: a horizontal adapting grating has no effect on vertical test gratings. There is partial interocular transfer of the after-effect.6. These findings provide further evidence that the visual system of man, like those of the cat and the monkey, contains neurones selectively sensitive to the orientation and dimensions of retinal images, and that these adaptable cells are actually involved in the encoding and perception of the size of simple patterns.

Motion and vision. I. Stabilized images of stationary gratings

To demonstrate that eye movements have profound effects on the sine-wave contrast threshold, the author uses a new method of stabilizing the retinal image, in which the Purkinje reflections from the eye move the stimulus pattern displayed on a CRT screen. Calibration of this compensatory motion is very critical; a gain error greater than 1% may produce significant destablization. Under optimum conditions, image stabilization elevates the subject's contrast threshold by a factor of about 20; it also produces after-images with resolution greater than 12 c/deg. These results compare favorably with those obtained by other methods.

The size-tuning of the face-distortion after-effect

Recently, Webster and MacLin demonstrated a face-distortion after-effect (FDAE) for both upright and inverted faces: adaptation to a distorted face makes a normal face appear distorted in the direction opposite to the adapting direction. Neurophysiological studies (e.g. Experimental Brain Research 65 (1986) 38) show that face-selective neurons in the superior temporal sulcus (STS) are remarkably size-invariant in their responses. If the site of adaptation underlying the FDAE is the homologous neuron population in human vision, then the FDAE should also be highly tolerant to changes in size between adapting and test faces. Here, we test this prediction. Observers were adapted to distorted upright/inverted faces of three different sizes (3.3 degrees x 3.7 degrees, 6.6 degrees x 7.5 degrees, and 13.1 degrees x 14.8 degrees ). For adapting faces of all three sizes, observers adjusted test faces of all three sizes until they appeared normal. Significant FDAEs were observed in all conditions. For both upright and inverted faces, FDAEs were approximately twice as strong when adapting and test faces were the same size than when they differed by even a single octave in size. The magnitudes of FDAEs were comparable for upright and inverted faces. The larger FDAEs for same-size adapting and test faces suggest that part of the FDAE derives from a neuron population with narrow size-tuning. However, the significant FDAEs obtained for adapting and test images differing by two octaves implicate a different neuron population with broad size-tuning, possibly the human homolog of the face-selective neuron population in monkey STS.

Influence of motion signals on the perceived position of spatial pattern

After adaptation of the visual system to motion of a pattern in a particular direction, a static pattern appears to move in the opposite direction-the motion aftereffect (MAE). It is thought that the MAE is not accompanied by a shift in perceived spatial position of the pattern being viewed, providing psychophysical evidence for a dissociation of the neural processing of motion and position that complements anatomical and physiological evidence of functional specialization in primate and human visual cortex. However, here we measure the perceived orientation of a static windmill pattern after adaptation to rotary motion and find a gradual shift in orientation in the direction of the illusory rotation, though at a rate much lower than the apparent rotation speed. The orientation shift, which started to decline within a few seconds, could persist longer than the MAE, and disappeared when the MAE was nulled by physical motion of the windmill pattern. Our results indicate that the representation of the position of spatial pattern is dynamically updated by neurons involved in the analysis of motion.

Color: a motion-contingent aftereffect

After human observers alternately view green stripes moving up and red stripes moving down for periods of 1/2 to 4 hours, they see a pink aftereffect when white stripes move up and a green aftereffect when white stripes move down. Longer exposures produce aftereffects which are visible 20 hours after stimulation. Thus, experience which pairs simple attributes (color and motion) of visual stimulation can result in a lasting modification of perception.

Orientation-selective adaptation and tilt after-effect from invisible patterns

Exposure to visual patterns of high contrast (for example, gratings formed by alternating white and black bars) creates after-effects in perception. We become temporarily insensitive to faint test patterns that resemble the pre-exposed pattern (such as gratings of the same orientation), and we require more contrast to detect them. Moreover, if the test pattern is slightly tilted relative to the pre-exposed one, this tilt may be perceptually exaggerated: we experience a tilt after-effect. Here we show that these visual after-effects occur even if the pre-exposed grating is too fine to be perceptually resolved. After looking at a very fine grating, so high in spatial frequency that it was perceptually indistinguishable from a uniform field, observers required more contrast to detect a test grating presented at the same orientation than one presented at the orthogonal orientation. They also experienced a tilt after-effect that depended on the relation of the test pattern's tilt to the unseen orientation of the pre-exposed pattern. Because these after-effects are due to changes in orientation-sensitive mechanisms in visual cortex, our observations imply that extremely fine details, even those too fine to be seen, can penetrate the visual system as far as the cortex, where they are represented neurally without conscious awareness.

Direction-specific adaptation in area MT of the owl monkey

Single neurons were recorded in owl monkey middle temporal visual cortex (MT). Directional neurons showed direction-selective adaptation to pattern motion: responses to motion in the preferred direction were reduced by adaptation to motion in the preferred direction and enhanced by adaptation in the opposite direction. Non-directional neurons did not show significant adaptation.

The effect of spatial frequency and contrast on visual persistence

The visual persistence of sinusoidal gratings of varying spatial frequency and contrast was measured. It was found that the persistence of low-contrast gratings was longer than that of high-contrast stimuli for all spatial frequencies investigated. At higher contrast levels of 1 and 4 cycles deg-1 gratings, a tendency for persistence to be independent of contrast was observed. For 12 cycles deg-1 gratings, however, persistence continued to decrease with increasing contrast. These results are compared with recently published data on other temporal responses, and are discussed in terms of the different properties of sustained and transient channels.

The neon color effect in the Ehrenstein illusion

Van Tuijl's neon color effect arises in the Ehrenstein figure if a colored cross is added such as to connect the black arms across the central gap. The effect consists of a circular veil of color in the illusory area and has the same hue as the inducing cross. The neon-like coloration is uniform, or when elicited by two color bipartite; it is strongest on backgrounds resembling the color of the cross. The effect cannot be attributed to chromatic aberration or eye movements. In foveal vision (and for red crosses) neon spreading is limited to gap sizes between 4 and 35 min of arc. Extrafoveally, gap sizes may be larger by a factor of two. Neon perception is enhanced by flicker and weakened if stimuli are oriented obliquely. It does not occur with dichoptic presentation. A maximum illusion requires that the Ehrenstein figure and cross are laterally and angularly aligned for good perceptual continuation. A neuronal origin by spreading and summation, together with cognitive processes, is proposed.