Visual evoked potentials (VEPs) were recorded from cat cortex (area 17) before, during and after application of the GABA blocker bicuculline (iontophoretic or topical). The stimuli comprised a test sinusoidal grating, and a mask grating oriented either parallel or orthogonal to the test. Both test and mask alternated in contrast at different temporal frequencies. VEPs were averaged in synchrony with the test contrast reversal, so the mask did not contribute directly to the averaged VEP response. Before application of bicuculline, both parallel and orthogonal masks attenuated the amplitude of VEPs and changed the phase response, but in different ways. Orthogonal masks lowered the slope of the contrast response curve without affecting extrapolated threshold, while parallel masks caused the curve to shift to the right. Orthogonal masks increased the phase advance, while parallel masks eliminated it. During application of bicuculline, neither parallel nor the orthogonal masks attenuated VEP amplitudes. The results suggest that although the mechanisms for the action of parallel and orthogonal masks are clearly distinct, both are mediated by the GABA-ergic inhibitory system. Given this evidence, measurement of VEP contrast response curves may provide a simple non-invasive technique for monitoring visual inhibition in humans.

Basketball is a popular sport in Australia. Although orofacial injuries are common, mouthguard (MG) wear in basketball appears to be low. The purposes of this study were: to measure mouthguard wear by basketball players before and after a promotional intervention; to assess players' knowledge of the value of mouthguards for prevention of injury; and to describe their experience of orofacial injury. Two questionnaires (baseline and follow-up) were administered to a convenience sample of 496 basketball players in Victoria, Australia. Players recruited were youths (12-15-year olds, n = 208) and adults (18 years and over, n = 288), from all basketball levels (social to elite). Completion of the baseline questionnaire was followed immediately by an intervention comprising written and verbal information, a mouthguard blank and instructions on mouthguard construction. The follow-up questionnaire was mailed to all respondents 10-12 weeks later; 135 youths (65%) and 157 adults (54%) completed this. Mouthguard wear at baseline was low but was more frequent at games (62%) than at training (25%). Despite 90% of players acknowledging the protective value of a mouthguard, wear by youths did not increase following the intervention, and wear by adults increased by only 14% for training and 10% at games. Previous orofacial injury was recorded at baseline by 23% of players, but few had requested compensation from Basketball Australia (youths, 17%; adults, 30%). Two predictor variables were statistically identified as related to mouthguard wear: previous orofacial injury and age group. Mouthguard wear was significantly more frequent amongst players with previous injury; such players were 2.76 times more likely to be wearers than those without previous injury. Youths were 2.31 times more likely to wear mouthguards than adults. Only 34 players (12% of respondents at follow-up) had a mouthguard constructed from the blank provided. Although youth and adult groups differed, the overall extent of mouthguard use was disappointingly low. Despite wide recognition of mouthguard value, the intervention had little effect on promoting their use.

After a preliminary study of visual evoked potentials (VEPS) to a test grating seen in the presence of masks at different orientations, psychophysical data are presented showing the effects of adaptation and of masking on thresholds for detecting the same test grating. The test is a vertical grating of spatial frequency 2 cycles per degree; adapting and masking gratings differ from the test either in orientation or in spatial frequency. The effects of adaptation and masking are explained by a single mechanism model that assumes: (i) adaptation and masking both alter the contrast response (or transducer) function of the mechanism that detects the test; (ii) masks, but not adaptors, stimulate the mechanism that detects the test; and (iii) a test is detectable when it raises response level by a constant amount. The model incorporates two distinct tuning functions, a broad adaptive contrast function and a narrow effective contrast function. It accounts adequately for all the data, including the location and size of the facilitative dip found in some masking functions, the constant slopes of the threshold elevation segments of adaptation functions and the varying slopes of masking functions. It also predicts the sometimes surprising joint effects of adaptation followed by masking and of two masks operating simultaneously.

No abstract

Using a contrast matching procedure, we measured the perceived contrast of vertical test gratings after adapting to other gratings of either vertical or horizontal orientation. The results show that both parallel and orthogonal adapting gratings reduce perceived contrast and do so proportionally more at low test contrasts than at high. The results are consistent with a single mechanism model proposed by Ross and Speed [(1991). Proceedings of the Royal Society (Series B), 246, 61-69] that assumes that adaptation to gratings repositions contrast-response transducer functions. They are not consistent with the notion of two different forms of adaptation, subtractive for parallel and multiplicative for orthogonal adaptors as proposed by Snowden and Hammett [(1992). Nature, 355, 248-250]. Nowhere is the reduction in perceived contrast by an orthogonal grating greater than that by a parallel grating of the same contrast. A direct comparison using two orthogonal adaptors confirms the greater potency of parallel adaptors, but also reveals interactions between the adaptors.