The orienting reflex to changes in a conceptual stimulus dimension

The above discussion must be viewed with caution, as a second interpretation of the findings may be made on the basis of the topography of the visual cortex. Due to the spatial representation of the retina in the visual cortex, we may assurne that the upper- and lower-field responses obtained in this study originated at distinctly different areas of the striate cortex within the calcarine fissure . Thus the poten tials recorded between the scalp and earlobe may have been affected by differences in the volume conduction characteristics of the brain in relation to the loci of origin of the evoked responses. Perhaps greater attenuation occurred in the volume conductor in relation to the site of origin of the upper-field responses and that these responses at their site of origin were actually equal to or greater than the lower-field responses occurring in the calcarine fissure. Jeffreys (1968) has reported some electrode location data which suggests that the topographical factor may be important. Work is under way in our laboratory to test further this topographical hypothesis. Even if the topographical explanation should have some validity in accounting for upper-Iower field differences, it seems an unlikely candidate for explaining the effects of check size on evoked potential . amplitude. There is no reason to assurne that the effects are due to differences in the origin of the evoked response within the calcarine fissure, since exactly the same retinal area within each field is stimulated with each checkerboard stimulus display. It seems equally unlikely that the shift in maximal response amplitude from larger to smaller checks as the stimulus display is moved from the lower to the upper field (Fig. 2) is due to differences in the loci of origin of the responses. The effects of check size may be more plausibly explained in terms of differences in the size of the centers of retinal receptive fields located in the retinal areas projecting respectively to the upper and lower visual fields. Harter & White (1970) have built a rather strong case for the possibility that cortical evoked responses may be related to the size of receptive field centers, as reflected in the single unit responses of animals. Recently Harter (in press) obtained an interaction between check size and retinal eccentricity which was predicted from the hypothesis that evoked potentials and size of receptive field centers are related. Assuming that the receptive field hypothesis has validity, the resuIts depicted in Fig. 2 suggest that the receptive field centers located in that part of the retina stimulated from within the lower field have an average size of approximately 40 min, Psychon. Sei., 1970, Vol. 2 (2) whereas those wh ich project to the upper field sublend about iO min of visual angle. Why the retina projecting to the upper field should have smaller receptive-field centers than the area projecting to the lower field is not entirely elear. One may speculate that flying or rapidly moving objects appearing in the upper field must be detected while relatively far away if the organism is to respond appropriately to them. Thus, the part of the visual system projecting to the upper field may be particuIarly attuned to "specks in the sky" subtending less than 30 min of visual angle. On the other hand, ground objects which are sufficiently elose to the organism to be p otentially threatening generally may subtend somewhat larger visual angles and therefore that part of the visual system projecting to the lower field developed a relatively high degree of reactivity to objects subtending 30 or more minutes of visual angle. REFERENCES EASON, R. G., ODEN, D., & WHITE, C. T. Visually evoked cortical potentials and reaction time in relation to site of retinal stimulation. EEG Clinical Neurophysiology, 1967,22,313-324. EASON, R. G., & WHITE, C. T. Averaged occipital responses to stimulation of sites in the nasal and temporal halves of the retina. Psychonomic Science, 1967,7,309-310. EASON, R. G., WHITE, C. T., & ODEN, D. Averaged occipital responses to stimulation of sites in the upper and lower halves of the retina. Perception & Psychophysics, 1967, 2, 423-425. HARTER, M. R. Evoked cortical responses to checkerboard patterns: Effect of check-size as a function of retinal eccentricity. Vision Research, in press. HARTER, M. R., & SUITT, C. D. Visually-evoked corticaI responses and pattern vision in the infant: A longitudinal study. Psychonomic Science, 1970, 18, 235-237. HARTER, M. R., & WHITE, C. T. Effects of contour sharpness and check-size on visuaIly evoked cortical potentials. Vision Research, 1968,8,701-71l. HARTER, M. R., & WHITE, C. T. Evoked cortical responses to checkerboard patterns: Effect of check-size as a function of visual acuity. EEG Clinical Neurophysiology, 1970, 28,48-54. JEFFREYS, D. A. Separable components of human evoked responses to spatially patterned visual fields. EEG Clinical Neurophysiology. 1968,24,596. (Abstract) JOHN, E. R., HERRINGTON, R. N., & SUTTON, S. Effects of visual fonn on the evoked response. Science, 1967, 155, 1436-1442. MacKA Y, D. M. Evoked brain potentials as indicators of sensory infonnation processing. Neurosciences Research Program Bulletin, 1969,7,211-216. RIETVELD, W. J., TORDOIR, W. E., HAGENOUW, J. R., LUBBERS, J. A., & SPOOR, Th. A. C. Visual evoked responses to blank and to checkerboard patterned flashes. Acta Physiol. Pharmacol. Neerl., 1967, 14, 259-285. SPEHLMANN, R. The averaged electrical responses to diffuse and to patterned light in the human. EEG ClinicaI Neurophysiology, 1965, 19, 560-569. The orienting reflex to changes in a conceptual stimulus dimension* R. M. YAREMKO, MARJORIE w. BLAIR, and BRUCE T. LECKART San Diego State College, San Diego, Calif. 92115 The galvanic skin response (GSR) component of the orienting reflex was recorded for four groups of 12 Ss each. They were given 10 habituation trials to black on white numeric stimuli presented in seriatum. On Trial 11 an out-of-sequence test stimulus (TS) was delivered and was followed by one additional in-sequenee number. The TS was either ±I or ±II pi aces out of sequence. It was found that GSR magnitude to the TS was primarily a function of the amount of disparity provided by the TS, irrespective of the direction of that disparity. Dishabituation to the subsequent in-sequence stimulus was not, however, clearly present. The results were related to Sokolov's neuronal model of habituation and to previous research. According to Sokolov (1963), magnitude of the orienting reflex (OR) is primarily a function of the amount of *Supported in part by a grant to the first author from the San Diego State College F oundation. Requests for reprints should be addressed 10 R. M. Yaremko, Department of Psychology. San Diego State College, San Diego. California 92115. change in the parameters of ambient stimulation when directional dynamogenic effects are not operative. Much support for this contenti (...truncated)