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Biomedical subjects

H M Buchanan-Smith

Publications and source records attributed to H M Buchanan-Smith.

14 recordsLinked to original sources
PubMed2005

Social interaction with non-averse group-mates modifies a learned food aversion in single- and mixed-species groups of tamarins (Saguinus fuscicollis and S. labiatus).

For social species, being a member of a cohesive group and performing activities as a coordinated unit appear to provide a mechanism for the efficient transmission of information about food. Social learning about food palatability was investigated in two captive primates, Saguinus fuscicollis and S. labiatus, which form stable and cohesive mixed-species groups in the wild. We explored whether an induced food aversion toward a preferred food is modified during and after social interaction with non-averse conspecifics or congeners. Sets of intra- and interspecific pairs were presented with two foods, one of which was considered distasteful by one of the pairs (the other was palatable), and their behavior was compared pre-interaction, during interaction, and post-interaction. For the aversely-conditioned individuals of both species, the change in social context corresponded to a change in their preference for the food that they considered unpalatable, regardless of whether they had interacted with a conspecific or congeneric pair, and the change in food preference was maintained post-interaction. In a control condition, in which averse individuals did not have the opportunity to interact with non-averse animals, S. fuscicollis sampled the preferred food, but not as quickly as when given the opportunity to interact. We conclude that the social learning demonstrated here may allow individual tamarins to track environmental change, such as fruit ripening, more efficiently than asocial learning alone, because social learners can more quickly and safely focus on appropriate behavior by sharing up-to-date foraging information. Furthermore, since the behavior of congeners, as well as conspecifics, acts to influence food choice in a more adaptive direction, social learning about food palatability may be an advantage of mixed-species group formation to tamarins of both species.

Analysis of Variance↗
PubMed2005

Color vision pigment frequencies in wild tamarins (Saguinus spp.).

The adaptive importance of polymorphic color vision found in many New World and some prosimian primates has been discussed for many years. Polymorphism is probably maintained in part through a heterozygote advantage for trichromatic females, as such individuals are observed to have greater foraging success when selecting ripe fruits against a background of forest leaves. However, recent work also suggests there are some situations in which dichromatic individuals may have an advantage, and that variation in color vision among individuals possessing different alleles may also be significant. Alleles that confer a selective advantage to individuals are expected to occur at a higher frequency in populations than those that do not. Therefore, analyzing the frequencies of color vision alleles in wild populations can add to our understanding of the selective advantages of some color vision phenotypes over others. With this aim, we used molecular techniques to determine the frequencies of color vision alleles in 12 wild tamarin groups representing three species of the genus Saguinus. Our results show that allele frequencies are not equal, possibly reflecting different selective regimes operating on different color vision phenotypes.

Animals↗
PubMed2000

The behavioural ecology of mixed-species troops of callitrichine primates.

This review summarizes information on the behavioural ecology of mixed-species troops (interspecific associations) formed by different species of callitrichines, small New World monkeys, in western and central Amazonia. The formation of mixed-species troops is an integral part of the biology of several species of this subfamily. Niche separation between associated species is obtained through vertical segregation which results in differences in the prey spectrum. The degree of niche separation is a predictor for the stability of mixed-species troops. Individuals may benefit from the formation of mixed-species troops through increased safety from predators, increased foraging efficiency, and/or increased resource defence. Costs of mixed-species troop formation are probably very low and mainly relate to patterns of interspecific behavioural interactions. We point to gaps in our knowledge and suggest pathways for future research into mixed-species troops.

Animals↗
PubMed1998

The influence of stimulus shape on orientation acuity.

Orientation acuity was estimated for vertical and oblique bar stimuli. Discrimination thresholds were affected by changes in the length and width of the targets, falling as bar length was increased and, conversely, rising as the bars were made wider. These changes are complimentary, in that overall discrimination performance can be predicted by a single measure of the orientation "entropy" of the target, namely the height-to-width ratio. The data provide support for a model of orientation coding where discrimination performance is not simply a reflection of the signal-to-noise ratio in single cells in the striate cortex.

Anisotropy↗
PubMed1997

The oblique effect in orientation acuity.

We have examined the dependence of rotational acuity on the orientation bandwidth of a stimulus using two-dimensional, band-pass filtered, spatial noise. Stimuli had a bandwidth of 0.5 octave of spatial frequency, centred at 5.0 cyc/deg, and an orientation bandwidth that covered the range from 0.0 to 25.0 deg. Thresholds were obtained on one principal (vertical), and one oblique axis (45 deg). It was found that acuity declined on both axes as bandwidth increased, in a manner that was compatible with simple statistical principles with virtually perfect sampling of the image. There was some evidence that the intrinsic noise is greater on the oblique axis than on the vertical, and that oblique axes are less densely sampled than the principal axes. These differences are small and are insufficient, either on their own or taken together to explain the oblique effect.

Humans↗
PubMed1996

Mechanisms specialized for the perception of image geometry.

Angle discrimination thresholds were obtained for V-shaped targets with a base angle of 90 deg at four different pattern orientations (0, 45, 90 and 135 deg). A comparison of these thresholds with the orientation discrimination thresholds for the single lines from which the patterns had been constructed, revealed that angle acuity cannot be predicted from component acuity. Angle acuity is finer than the corresponding orientation acuity in all cases and does not exhibit the pronounced oblique effect that is found for orientation discrimination. Other experiments showed that acuity for pattern angle depends critically on base angle, with minima close to 0, 90 and 180 deg. The shape and amplitude of this function are independent of pattern orientation. It was found that the angle acuity was unaffected by excluding a large portion of the target in the region of the vertex, and that the pattern of dependence of acuity on angle changed radically when the target was reduced ultimately to three blobs that defined the cardinal points of the stimulus. The data suggest that when the target comprises line segments, angle discrimination is not limited by noise that arises at early levels of processing and that angle perception is mediated by mechanisms that are specialized for the perception of image geometry. An opponent process model, that is based on the combined outputs of just two types of filter, is proposed as the basis for the perception of image geometry. This type of system is appropriate for computing one of the differential invariants in an optic flow field.

Discrimination, Psychological↗
PubMed1994

Changes in the perceived direction of drifting plaids, induced by asymmetrical changes in the spatio-temporal structure of the underlying components.

When a plaid pattern with symmetrical velocity components (Type I) is changed to a plaid pattern with asymmetrical velocity components (Type IA), the overall direction of drift appears to undergo a rotation without any other change to the spatial parameters of the components. This change in the perceived drift direction can be induced by altering either the temporal frequency of the components or by altering their spatial frequency. In separate experiments, we have estimated the magnitude of the temporal and spatial frequency thresholds that are necessary to create a liminal change in direction of this type. The results from both temporal and spatial frequency experiments are closely similar. We find that liminal rotations can be induced by changes in the spatio-temporal structure of the sine-wave grating components that are undetectable when these components are presented in isolation. Further, we find that the "velocity threshold for direction" is not a constant factor, but critically depends on the relative orientation of the two elements that form the plaid. Forced-choice experiments were also conducted to estimate the extent of the apparent rotation of the plaid pattern for differing levels of asymmetry in the spatial frequency and temporal frequency of the components. The magnitude of the pattern rotation is predicted by a model of motion direction that encodes the successive displacements of the intersections of the gratings. Finally, we demonstrate that the velocity thresholds for perceived rotation exhibit a meridional anisotropy that depends on the direction of drift of the overall pattern and not on the orientation of the components.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1994

Evidence for separate, task dependent noise processes in orientation and size perception.

Spatial acuity was estimated as the minimum difference in target spatial frequency which could act as a cue in a double concurrent, single interval orientation discrimination task. Experiments were conducted with vertical and oblique (45 deg) targets at three different spatial frequencies covering a range of two octaves. Frequency thresholds were found to be lower than those estimated by conventional methods by a factor of nearly 50%. They followed Weber's law for target frequency and were independent of target orientation. The orientation thresholds exhibited a normal oblique effect, and were not affected by the simultaneous judgements of target frequency. The data support the contention that the neural codes for image size and image orientation are segregated at an early stage of processing, and that the main source of noise that limits the precision of coding in these domains exists beyond the level of early mechanisms.

Differential Threshold↗
PubMed1993

Orientation acuity for sine-wave gratings with random variation of spatial frequency.

Orientation discrimination thresholds were estimated for sine-wave grating stimuli using a two-alternative forced-choice procedure on the two principal, and the two main oblique axes. The two discriminanda within any trial were subject to independent random variation of their spatial frequency. The variation of spatial frequency was defined by a Gaussian probability density function. Thresholds were obtained for five different depths of frequency variation within the range 0.0-1.0 octave. We find that orientation acuity on any of the axes tested is unaffected by variation in the spatial frequency of the targets. The data support the hypothesis that orientation and spatial frequency are independent and orthogonal neural codes.

Discrimination, Psychological↗
PubMed1993

Anisotropic axes in orientation perception are not retinotopically mapped.

The effect of the posture of an observer on the pattern of meridional anisotropy for orientation acuity was examined. Orientation acuity was estimated for the two principal and for one oblique axis with three bodily postures: normal vertical sitting, vertical sitting with the head rotated to 45 degrees from vertical, and prone. The test stimuli were sine-wave gratings covering a range of two octaves of spatial frequency, thin lines, and step edges. Acuity estimates were obtained by two different psychophysical procedures: a single-interval two-alternative forced choice, and a two-interval two-alternative forced choice. Both experimental procedures revealed a strong interaction between the shape of the function that describes the meridional variation of orientation acuity and the posture of the observer. The data are incompatible with models of the oblique effect in orientation perception that are based on a meridional anisotropy in neural properties early in the visual hierarchy.

Anisotropy↗
PubMed1992

Directional acuity for drifting plaids.

Orientation discrimination thresholds were estimated for symmetrical "plaids", constructed by the super-imposition of two, independent drifting sine-wave gratings of 2.5 c/deg. Experiments were conducted when the apparent direction of drift was on the two principal meridians (vertical and horizontal), and when the apparent direction of drift was at the two oblique orientations (45 and 135 deg). Acuity for the direction of drift for these stimuli is of the same order of precision as orientation acuity for static or drifting gratings, and exhibits a meridional anisotropy that favours the principal meridians. This anisotropy follows a pattern that is determined by the apparent direction of drift, and not the orientation of the underlying elements. Acuity for orientation is lowest for oblique drift directions, even though both of the elements are oriented on the principal meridians. This finding was confirmed when the orientation of the underlying elements was subject to a random variation. The results are not easily accommodated by models that propose that the individual elements of the plaid are analysed separately. Second, the data are incompatible with models of the oblique effect of orientation discrimination that are based on an axis dependent, differential sampling of the retinal image.

Discrimination, Psychological↗
PubMed1992

Orientation acuity estimated with simultaneous and successive procedures.

Orientation discrimination thresholds for medium contrast sinewave grating stimuli were estimated by a forced-choice procedure for the two principal, and the two main oblique axes. The results obtained for simultaneous presentation with a centre/surround stimulus arrangement are compared with the threshold estimates obtained when the two stimuli to be discriminated do not overlap in time. We find that orientation acuity depends on the temporal relationship of the discriminanda. Estimates of orientation threshold with simultaneous presentation are lower on all axes tested than those obtained with successive presentation; the greatest difference in orientation acuity was found at obliques. The data support the hypothesis that the meridional anisotropy in orientation discrimination that is found with conventional, two-interval forced-choice methods is not entirely attributable to anatomical and physiological changes at the level of the striate cortex.

Discrimination Learning↗
PubMed1990

Recognition of stimulus orientation.

The threshold for detecting a deviation in the orientation of a test stimulus from vertical and from horizontal was measured with a single test interval. The test stimuli were step edges, thin lines, and sine wave gratings of 2.5, 5.0 and 10.0 c/deg. Recognition thresholds were found to be independent of the frequency content and nature of the test stimulus. The average threshold value on the principal meridians was found to be approx. 0.6 deg. Additional testing was conducted with two interval forced choice designs, both with and without random perturbation of the orientation of the stimuli. Random perturbation of orientation reduces acuity estimates. The results are compared with those obtained by testing on the oblique meridia. We conclude that the meridional anisotropy of orientation discrimination, which favours the vertical and horizontal is a result, in part, of the influence of non-visual mechanisms.

Adolescent↗