Motion aftereffect transfer in the monofixation syndrome.

If one adapts to a moving repetitive stimulus of stripes that is suddenly stopped, the stripes will appear to move backward. This apparent backward motion is the motion aftereffect (MAE), and its duration is a measure of its magnitude. If one eye adapts to the moving stimulus and the other eye experiences the aftereffect to the stationary stimulus, the aftereffect has been transferred from one eye to the other and is termed the interocular transfer of the MAE. Experimental evidence indicates that the degree of MAE transfer correlates with clinical binocularity. This study compares the MAE transfer in six subjects with the monofixation syndrome to five normal subjects. The stimuli used are sinusoidal stripes generated on two cathode-ray tubes, subtending either 8 degrees or 2 degrees of visual angle with a periodicity of either 0.5 or 3 cycles/degree presented haploscopically. Subjects with the monofixation syndrome differed significantly from normal subjects in the amount of MAE transferred, implying a lack of central neuronal connections in addition to those mediating conscious central fusion in clinical sensory testing.

Interocular transfer of the motion aftereffect in strabismus.

The interocular transfer of the motion aftereffect (MAE) was measured in three groups of strabismic subjects (six with monofixation, nine with alternation, four with anomalous retinal correspondence) and compared with a group of four normal subjects. The duration of the MAE was measured using sinusoidal gratings of 0.5 c/deg subtending a visual angle of 8. Contrary to previous findings, a substantial amount of MAE transfer was found in some stereoblind individuals with strabismus. The mean amount of transfer for each group was found to correlate with binocularity; the order of decreasing transfer being: normal, monofixation, alternation, and anomalous retinal correspondence (ARC). This reduction in transfer which accompanied the loss of binocularity was asymmetric, that is, right-left transfer did not equal left-right transfer. The amount of asymmetry was found to correlate inversely with the amount of transfer. The direction in which the greatest transfer occurred did not correlate with visual acuity. These data substantiate the overall poor binocularity in subjects with ARC, and the relatively good binocularity of subjects with monofixation. Furthermore, the substantial amount of transfer found in subjects with alternating strabismus may be a measure of potentially achievable binocularity.

Influence of the spatial periodicity of moving gratings on motion response.

The present experiments examine the suprathreshold response of the motion or direction-selective portion of the human visual system by means of the motion aftereffect (MAE). The MAE was measured as a function of the contrast and spatial frequency of moving sinusoidal gratings. For spatial frequencies less than 1 cy/deg, the MAE speed was found to increase linearly with log contrast up to 80%. For spatial frequencies greater than 1 cy/deg, the rate of increase of the MAE speed with log contrast was not found to be linear over the entire range of contrast. The nonlinearity was greatest for the 8 and 10 cy/deg gratings, which showed very little increase in MAE speed with contrast above 25%. We conclude that the direction-specific mechanisms in human vision show a more limited contrast response to the high spatial frequencies than does the visual system as a whole.

Visual evoked response as a function of grating spatial frequency.

Transient visual evoked responses (VER's) to the appearance-disappearance of sinusoidal gratings have been investigated for a range of spatial frequencies. Contrary to the results of previous studies, the results show that the transient VER consists of a relatively simple waveform that is most easily characterized by the initial negative peak (N1) whose latency and amplitude vary with the contrast and spatial frequency of the grating. At spatial frequencies less than 3 cycles/degree (c/d) an additional short latency component appears in the response. This component is maximum at 1 to 2 c/d, saturates at low contrast, and is insensitive to the precise position of the grating on the retina. The results are related to the properties of transient and sustained channels assumed to exist in the human visual system.

Recovery from adaptation to moving gratings.

Short-term adaptation to moving sinusoidal gratings results in a motion aftereffect which decays in time. The time decay of the motion aftereffect has been measured psychophysically, and it is found to depend on (i) the spontaneous recovery from the adapted state, and (ii) the contrast of the test grating. We have measured the decays for various test conditions. An extrapolation of the measurements allows us to obtain a decay which represents the time course of the spontaneous recovery of the direction-sensitive mechanisms.