Some comments on Thomas’ (1977) reciprocal inhibitory model for monocular rivalry
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Perception & Psychophysics 1978, Vol. 23 (6),553-554 Somecomments on Thomas' (1977) reciprocal inhibitory model for monocular rivalry PETER WALKER Preston Polytechnic, Preston PR1 7DP, England Thomas (1977) has recently discussed the relation- ship between the spatial frequency of superimposed grating patterns and their rate of monocular rivalry. On the basis of this relationship and a number of other observations, he concludes that reciprocal (lateral) in- hibition underlies monocular rivalry. On several counts, however, such a hypothesis is not only unjustified but is contradicted by the evidencethat is available. The first point that may be made is. that it is by no means clear how reciprocal inhibition might begin to explain even the most fundamental features of monocular rivalry, viz, the dominance of one pattern and the alternation in dominance. Why, for example, when one pattern is dominant, should the other pattern ever return to dominance? Indeed, if one pattern is "stronger" than the other, why should it ever be rendered nondominant? More particularly, Thomas draws heavily on his own findings, and those of Kitterle, Kaye, and Nixon (1974), which show an inverted U-shaped function relating the rate of pattern alternation with the spatial frequency of one or both gratings-i-the optimum fre- quency being approximately 5 c/deg. Thomas argues that this result supports the reciprocal inhibition model because 5 c/deg patterns are particularly effective stimuli for the visual system, and hence would create powerful reciprocal inhibition between their respective neural channels. More powerful inhi- bition would, according to Thomas, bring about a faster rate of alternation. Similarly, Kitterle et al, (1974) argue that obliquely oriented gratings alternate less frequently than vertical and horizontal gratings because they are less effective stimuli for the visual system, inducing less lateral inhibition. While Kitterle et al. refer to other researches which justify the assumption regarding the relative effectiveness of oblique vs. horizontal/vertical gratings, Thomas does not draw on other data to confirm the relative effectiveness of 5 c/deg, To this extent, his argument is rather circular. This aside, however, it is still unclear how these effects are to be accommodated by reciprocal inhibition. For example, if both stimuli are increased in "strength," not only might the strength of inhibition between them be made more powerful, but, by definition, the strength of activa- tion of the channels themselves would also increase. Could not this latter increase more than compensate the former increase, so that less rivalry would be predicted? Clearly, any model based on reciprocal inhibition must specify the nature of a presumably complex relationship between the strength of direct activation of the individual channels and the strength of their reciprocal inhibition. Thomas also mentions in support of his hypothesis that the rate of alternation decreases as the spatial frequency of the grating patterns is made increasingly different. His argument is that providing such a difference in spatial frequency should result in fewer alternations because of a weakened inhibitory inter- action. Unfortunately, Thomas' own study and that of Kitterle et al., from which support is derived, con- found the difference in spatial frequency with the absolute values for these frequencies. If one next. considers other evidence regarding monocular rivalry, one discovers further grounds for rejecting Thomas' hypothesis. Thomas draws our attention to the fact that while orientation-sensitive channels respond to orienta- tions that deviate by as much as 10°_15° from their preferred orientation, monocular rivalry is not observed until the orientations of two superimposed gratings differ by a least 15°_20°. The suggestion is that only when such an orientation difference occurs does lateral inhibition between the corresponding channels arise. The evidence indicates, however, that lateral inhibition may be observed between channels whose preferred orientations are within 15°-20° of each other (Blakemore & Tobin, 1972; Carpenter & Blakemore, 1973). More important, however, if one considers the effect of increasing the orientational separation of superimposed gratings beyond 20°, then, while the strength of the lateral inhibition between their respective channels decreases (Blakemore & Tobin, 1972; Carpenter & Blakemore, 1973), the rate of monocular rivalry actually increases (Campbell, Gilinsky, Howell, Riggs, & Atkinson, 1973). There is, then, a negative correlation between the occurrence or rate of monocular rivalry and the magnitude of the underlying lateral inhibition (see Walker, 1976). Consistent with this is the fact, acknowledged by Thomas, that the rate of monocular rivalry is greater for differently colored gratings (e.g., one in red and the other in green) (Rauschecker, Campbell, & Atkinson, 1973). This also contradicts Thomas' hypothesis, since lateral inhibition between orientation- sensitive channels is selective to wavelength(Lovegrove, 1977). In conclusion, there seems to be little support for Thomas' reciprocal inhibition approach to visual rivalry. Rather, the evidence would appear to support the counter proposal that the readiness with which two patterns engage in rivalry is positively correlated with the extent to which their respective channels are 553
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Transcript of Some comments on Thomas’ (1977) reciprocal inhibitory model for monocular rivalry
Perception & Psychophysics1978, Vol. 23 (6),553-554
Somecomments on Thomas' (1977)reciprocal inhibitory model
for monocular rivalry
PETER WALKERPreston Polytechnic, Preston PR1 7DP, England
Thomas (1977) has recently discussed the relationship between the spatial frequency of superimposedgrating patterns and their rate of monocular rivalry. Onthe basis of this relationship and a number of otherobservations, he concludes that reciprocal (lateral) inhibition underlies monocular rivalry. On severalcounts,however, such a hypothesis is not only unjustified butis contradicted by the evidencethat is available.
The first point that may be made is. that it isby no means clear how reciprocal inhibition mightbegin to explain even the most fundamental featuresof monocular rivalry, viz, the dominance of onepattern and the alternation in dominance. Why, forexample, when one pattern is dominant, should theother pattern ever return to dominance? Indeed, ifone pattern is "stronger" than the other, why shouldit ever be rendered nondominant?
More particularly, Thomas draws heavily on hisown findings, and those of Kitterle, Kaye, and Nixon(1974), which show an inverted U-shaped functionrelating the rate of pattern alternation with the spatialfrequency of one or both gratings-i-the optimum frequency being approximately 5 c/deg. Thomas arguesthat this result supports the reciprocal inhibitionmodel because 5 c/deg patterns are particularlyeffective stimuli for the visual system, and hencewould create powerful reciprocal inhibition betweentheir respective neural channels. More powerful inhibition would, according to Thomas, bring about afaster rate of alternation. Similarly, Kitterle et al,(1974) argue that obliquely oriented gratings alternateless frequently than vertical and horizontal gratingsbecause they are less effective stimuli for the visualsystem, inducing less lateral inhibition. While Kitterleet al. refer to other researches which justify theassumption regarding the relative effectiveness ofoblique vs. horizontal/vertical gratings, Thomasdoes not draw on other data to confirm the relativeeffectiveness of 5 c/deg, To this extent, his argumentis rather circular. This aside, however, it is stillunclear how these effects are to be accommodated byreciprocal inhibition. For example, if both stimuliare increased in "strength," not only might thestrength of inhibition between them be made morepowerful, but, by definition, the strength of activation of the channels themselves would also increase.Could not this latter increase more than compensatethe former increase, so that less rivalry would be
predicted? Clearly, any model based on reciprocalinhibition must specify the nature of a presumablycomplex relationship between the strength of directactivation of the individual channels and the strengthof their reciprocal inhibition.
Thomas also mentions in support of his hypothesisthat the rate of alternation decreases as the spatialfrequency of the grating patterns is made increasinglydifferent. His argument is that providing such adifference in spatial frequency should result in feweralternations because of a weakened inhibitory interaction. Unfortunately, Thomas' own study and thatof Kitterle et al., from which support is derived, confound the difference in spatial frequency with theabsolute values for these frequencies.
If one next. considers other evidence regardingmonocular rivalry, one discovers further groundsfor rejecting Thomas' hypothesis.
Thomas draws our attention to the fact that whileorientation-sensitive channels respond to orientations that deviate by as much as 10°_15° from theirpreferred orientation, monocular rivalry is notobserved until the orientations of two superimposedgratings differ by a least 15°_20°. The suggestion isthat only when such an orientation difference occursdoes lateral inhibition between the correspondingchannels arise. The evidence indicates, however, thatlateral inhibition may be observed between channelswhose preferred orientations are within 15°-20° ofeach other (Blakemore & Tobin, 1972; Carpenter &Blakemore, 1973). More important, however, if oneconsiders the effect of increasing the orientationalseparation of superimposed gratings beyond 20°,then, while the strength of the lateral inhibitionbetween their respectivechannels decreases (Blakemore& Tobin, 1972; Carpenter & Blakemore, 1973), therate of monocular rivalry actually increases (Campbell,Gilinsky, Howell, Riggs, & Atkinson, 1973). Thereis, then, a negative correlation between the occurrenceor rate of monocular rivalry and the magnitude ofthe underlying lateral inhibition (see Walker, 1976).Consistent with this is the fact, acknowledged byThomas, that the rate of monocular rivalry is greaterfor differently colored gratings (e.g., one in red andthe other in green) (Rauschecker, Campbell, &Atkinson, 1973). This also contradicts Thomas'hypothesis, since lateral inhibition between orientationsensitive channels is selective to wavelength (Lovegrove,1977).
In conclusion, there seems to be little supportfor Thomas' reciprocal inhibition approach to visualrivalry. Rather, the evidence would appear to supportthe counter proposal that the readiness with whichtwo patterns engage in rivalry is positively correlatedwith the extent to which their respective channels are
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554 WALKER
independent and do not interact via lateral inhibition(Walker, 1976, in press).
REFERENCES
BLAKEMORE, c., & TOBIN, E. A. Lateral inhibition betweenorientation detectors in the eat's visual cortex. ExperimentalBrain Research, 1972, 15, 439-440.
CAMPBELL, F. W., GILINSKY, A. S., HOWELL, E. R., RIGGS,L. A.,& ATKINSON, J. The dependence of monocular rivalry onorientation. Perception, 1973, 2, 123-125.
CARPENTER, R. H. S., & BLAKEMORE, C. Interactions betweenorientations in human vision. Experimental Brain Research,1973, 18, 287-303.
KITTERLE, F. L., KAYE, R. S., & NIXON, H. Pattern alternation:Effects of spatial frequency and orientation. Perception &Psychophysics, 1974, 16, 543-547.
loVEGROVE, W. Inhibition between channels selective to contourorientation and wavelength in the human visual system. Perception & Psychophysics, 1977, 22,49-53.
RAUSCHECKER, J. P. J., CAMPBELL, F. W., & ATKINSON, J.Colour opponent neurons in the human visual system. Nature,1973, 245,42-43.
THOMAS, J. A reciprocal inhibitory model for monocular patternalternation. Perception & Psychophysics, 1977, 22,310-312.
WALKER, P. The perceptual fragmentation of unstabilized images.Quarterly Journal ofExperimental Psychology, 1976, 28, 35-45.
WALKER, P. Orientation-selective inhibition and binocular rivalry.Perception, 1978, 7, 207-214.
(Received for publication April 13, 1978;accepted April 13, 1978.)
