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Notes: Abstract We have studied the IMG functions associated with visual detection of targets moving across a background structure (Barbur and Ruddock, 1980a) in four subjects, each possessing a different visual abnormality. Results for subject J.F., a tritanope, show that the low-frequency IMG response associated with the blue-sensitive (π1) mechanism (Barbur and Ruddock, 1980b) is absent in his case, thus providing confirmation that it is π1 which gives rise to the low-frequency response. We establish that, for an albino subject, D.C., the low visual acuity for resolution of gratings arises post-receptorally and show that the IMG function is restricted to the same low spatial frequency range as the visual acuity data. Visual responses to moving targets can be elicited from the blind hemi-field of a hemianopic subject, G. Y., but there is no IMG function associated with these responses, which also exhibit very low spatial resolution. Finally, IMG functions, measured with longwavelength background gratings, are grossly abnormal for a subject, M. W., who possesses a central colour vision defect in detection of long-wavelength targets. On the basis of these experimental data, we argue that the IMG functions involve central response mechanisms, and we conclude that these functions provide a new method for the investigation of defective visual systems.

Notes: Abstract Visual detection of targets moving against structured background fields has been studied with near-monochromatic stimuli, selected so as to isolate the different increment threshold spectral response mechanisms. It is shown that for foveal vision, the red-and green-sensitive mechanisms (π5 and π4 respectively) yield IMG functions (Barbur and Ruddock, 1980), similar to those found with white light. In contrast, the blue-sensitive (π1) mechanism yields a low-frequency IMG response quite unlike that found for the other mechanisms. There is also considerable variation between subjects in this case. Measurements taken 30° off-axis with low (1.4 log trolands) background illumination level, yield a low frequency response IMG function for both rod and cone spectral mechanisms, similar to those found with white light stimuli. At high illumination levels (〉2.2 log trolands), the IMG function for the π5-mechanism is shifted to higher spatial frequencies, as is also observed with white light stimuli. A wavelength-selective binocular interaction effect, manifested in the detection of moving targets, is also described, and it is suggested that this may be of value in the study of defective colour vision.

Notes: Abstract The contrast threshold elevation effect has been measured for one dimensional (grating) and for two dimensional (spot) stimulus patterns. It has been shown previously (Burton and Ruddock, 1978) that such stimuli elicit, respectively, non-length-selective and length-selective adaptation effects. It is established that, unlike the frequency shift effect, the contrast threshold elevation effect is sensitive to the width of the light but not to that of the dark elements of the stimulus patterns. Adaptation to spot patterns elicits a significant threshold elevation for detection of both spot and grating test stimuli, but only under monoptic viewing conditions. The experimental findings are summarized in a block-diagram and it is shown that adaptation to grating patterns is successfully described by the spatial frequency response data given by Maudarbocus and Ruddock (1973).

Notes: Abstract The investigation is concerned with the processing of light and dark bar patterns by the human visual system. A response phenomenon, the frequency shift effect, is studied with particular attention being given to the most suitable parametric description. Previous investigations of the effect showed that the perceived spatial frequency of a test grating pattern can be changed, transiently, after a period of adaptation to a grating of different frequency. In the present work it is shown that the perceived width of the bars of one polarity (i.e. light or dark) in the test grating can be changed independently of the width of bars of the opposite polarity both in the test grating and in the adaptation grating. It is shown that there is no simple interpretation of this result in terms of the spatial frequency spectra of the gratings. It is suggested that the human visual system processes positive and negative contrast polarities in independent channels. The results are discussed in relation to the on- and off-centre receptive field characteristics obtained in microelectrode recordings from vertebrate visual systems.

Notes: Abstract Threshold illumination levels, I t , for visual detection of white light targets, moving across spatially structured background fields, have been measured and it is shown that, for a given background illumination level, I t depends upon the spatial characteristics of the background field structure. Thus, with the background composed of a square-waveform grating of fundamental spatial frequency f cycles/deg, I t is maximum for an intermediate value of f, and falls as f increases or decreases from this value. The relationship between I t and f characterizes the interaction between Movement detection and the background grating, and is designated the IMG function. The parametric properties of the IMG functions are described and it is established that the mechanisms which give rise to these functions are sensitive to the movement, but not the spatial structure of the target. They correspond, therefore, to the movement-sensitive Y-type mechanisms, observed in electrophysiological studies of cat and primate visual pathways. The spatial distribution of sensitivity associated with the IMG functions has been computed by 2-D transform methods, the computation yielding circularly symmetric, centre-surround antagonistic “receptive field” distributions.

Notes: Abstract 1. We have studied visual detection of a circular target moving across a spatially and/or temporally modulated background. Illumination, I t , for threshold detection of the target has been measured as a function of background modulation frequency and changes in I t associated with background modulation provide a means of determining the frequency response characteristics of visual channels. 2. Temporal frequency responses obtained with temporally modulated, spatially uniform backgrounds have pass-band characteristics and the temporal frequency for peak response increases with increase in mean background illumination. These temporal frequency responses resemble those of the de Lange (1954) filter, but the latter incorporates the incremental thresholds for steady backgrounds. 3. The amplitude of this temporal response saturates at low (∼40%) background modulation, decreases to zero as the target velocity falls to zero, and is maximum for a circular target of diameter 2°. 4. The spatial characteristics of this temporal filter were measured with a background field consisting of alternate steady and flickering bars. The resulting spatial frequency curve peaks at 1 cycle deg-1 for all background illuminations and is independent of the background grating orientation. This spatial response differs significantly from the IMG spatial functions observed with a background grating (Barbur and Ruddock, 1980). 5. The spatial and temporal responses reviewed above exhibit similar parametric variations and we therefore associate them with a single spatiotemporal filter, ST2. 6. A second temporal response, with low-pass frequency characteristics, was observed with a background field consisting of two matched gratings, presented in spatial and temporal antiphase. This response has parametric properties similar to those of the IMG spatial response described previously by Barbur and Ruddock (1980), thus we associated the two sets of data with a single spatio-temporal filter, ST1. 7. We show that the ST2 responses can be obtained by combining ST1 responses, and we present a network incorporating the two filters. 8. We review other psychophysical studies which imply the activity of two spatio-temporal filters with properties of the kind revealed in our studies. We argue that filter ST1 has properties equivalent to those of X-type and filter ST2 has properties equivalent to those of Y-type electrophysiological mechanisms.

Notes: Abstract 1. We have used the psychophysical methods described in the first paper of this series (Holliday and Ruddock, 1983) to determine selected spatial and temporal response characteristics of the ST1 and ST2 filters for subjects suffering visual defects. Data are given for 19 amblyopes, an albino and a hemianope, and comparison data are also given for a number of subjects with normal vision. 2. The ST1 spatial responses for both the “normal” and “amblyopic” eyes of 12 convergent strabismic amblyopes are displaced to low spatial frequencies compared to the normal curve, which implies that there is a loss of fine spatial tuning. In all but one subject, the curve for the “amblyopic” eye peaks at a spatial frequency lower than that for the “normal” eye, thus the former deviates further from the normal pattern than the latter. 3. The ST1 spatial responses of 6 refractive amblyopes are also displaced to the low frequency side of the normal curve, although on average the shift is smaller than in the case of the strabismic amblyopes. For each subject, the response curve of the “amblyopic” eye peaks at a lower spatial frequency than does that for the “normal” eye. 4. ST1 spatial responses were measured for targets located up to 30° off-axis along the horizontal meridian and sample data are given for one strabismic and one refractive amblyope and for two normal subjects. It is concluded from these data that the changes in the spatial responses associated with amblyopia do not simply reflect eccentric fixation of the target. 5. The ST2 spatial response was measured for the “normal” and “amblyopic” eyes of 9 amblyopes, (7 stabismic and 2 refractive). There is no significant difference between the average amblyopic response and that of, normal subjects, and only in one case does the response for an “amblyopic” eye peak at a frequency lower than the peak frequency for normal vision. 6. The ST2 temporal response for 9 amblyopes shows no systematic deviations from the normal response. 7. For the albino, both the ST1 and ST2 spatial responses peak at around 0.3 cycles deg-1, and both curves are displaced considerably to the low spatial frequency side of the normal ST2 spatial response. The albino's ST2 temporal response is essentially normal. 8. Measurements for the hemianope's “blind” hemifield under conditions appropriate to the isolation of the ST1 and ST2 spatial responses reveal no tuning curves. The ST2 temporal response for the “blind” hemifield, however, is of large amplitude, with a peak at 2 Hz, well below the normal frequency response peak. 9. It is argued that the loss of fine spatial tuning which occurs in the ST1, but not the ST2, spatial responses of the amblyopes is consistent with the sequential organisation of these two filter classes proposed by Holliday and Ruddock (1983). Further, for the only two subjects whose ST2 spatial response curves are displaced to abnormally low frequencies (the albino and a strabismic amblyope) the ST1 spatial response is shifted to low spatial frequencies compared to the normal ST2 curve. This is also consistent with Holliday and Ruddock's (1983) model.

Notes: [Auszug] Our studies concerned a special class of stimulus patterns? one-dimensionally periodic distributions of light which, because of their alternating light and dark bar structure, are called grating stimuli (Fig. 1 inset). The visual detectability of such grating stimuli is frequently specified by the ...

Notes: [Auszug] The representation of the visual field on the human striate cortex was first established from the study of patients who developed permanent blindness in localized areas following gunshot wounds to the striate cortex1. However, visual discriminations in response to light stimulation within 'blind' ...