Biomedical subjects

Greet Kayaert

Publications and source records attributed to Greet Kayaert.

3 recordsLinked to original sources

PubMed2005

Tuning for shape dimensions in macaque inferior temporal cortex.

It is widely assumed that distributed bell-shaped tuning (e.g. Radial Basis functions) characterizes the shape selectivity of macaque inferior temporal (IT) neurons, analogous to the orientation or spatial frequency tuning found in early visual cortex. Demonstrating such tuning properties requires testing the responses of neurons for different values along dimensions of shape. We recorded the responses of single macaque IT neurons to variations of a rectangle and a triangle along simple shape dimensions, such as taper and axis curvature. The neurons showed systematic response modulation along these dimensions, with the greatest response, on average, to the highest values on the dimensions, e.g. to the most curved shapes. Within the range of values tested, the response functions were monotonic rather than bell-shaped. Multi-dimensional scaling of the neural responses showed that these simple shape dimensions were coded orthogonally by IT neurons: the degree and direction of responses modulation (i.e. the increase or decrease of responses along a dimension) was independent for the different dimensions. Furthermore, for combinations of curvature-related and other simple shape dimensions, the joint tuning was separable, that is well predicted by the product of the tuning for each of the dimensions. The independence of dimensional tuning may provide the neural basis for the independence of psychophysical judgements of multidimensional stimuli.

Action Potentials↗

PubMed2004

Representation of regular and irregular shapes in macaque inferotemporal cortex.

We determined the degree to which the response modulation of macaque inferior temporal (IT) neurons corresponds to perceptual versus physical shape similarities. IT neurons were tested with four groups of shapes. One group consisted of variations of simple, symmetrical (i.e. regular) shapes that differed in nonaccidental properties (NAPs, i.e. viewpoint-invariant), such as curved versus straight contours. The second and third groups were composed of, respectively, simple and complex asymmetrical (i.e. irregular) shapes, all with curved contours. A fourth group consisted of simple, asymmetrical shapes, but with straight (corners) instead of curved contours. The neural modulations were greater for the shapes differing in NAPs than for the shapes differing in the configuration of the convexities and concavities. Multidimensional scaling showed that a population code of the neural activity could readily distinguish the four shape groups. This pattern of neural modulation was strongly manifested in the results of a sorting task by human subjects but could not be predicted using current image-based models (i.e. pixel energies, V1-like Gabor-jet filtering and HMAX). The representation of shape in IT thus exceeds a mere faithful representation of physical reality, by emphasizing perceptually salient features relevant for essential categorizations.

Algorithms↗

PubMed2003

Shape tuning in macaque inferior temporal cortex.

Neurons in the inferior temporal cortex (IT) of the macaque fire more strongly to some shapes than others, but little is known about how to characterize this shape tuning more generally, because most previous studies have used somewhat arbitrary variations in the stimuli with unspecified magnitudes of the changes. The present investigation studied the modulation of IT cells to nonaccidental property (NAP, i.e., invariant to orientations in depth) and metric property (MP, i.e., depth dependent) variations of dimensions of generalized cones (a general formalism for characterizing shapes hypothesized to mediate object recognition). Changes in an NAP resulted in greater neuronal modulation than equally large pixel-wise changes in an MP (including those consisting of a rotation in depth). There was also precise and highly systematic neuronal tuning to the quantitative variations of MPs along specific dimensions to which a neuron was sensitive. The NAP advantage was independent of whether the object was composed of only a single part or had two parts. These findings indicate that qualitative shape changes such as NAPs help explain the surplus amount of IT shape sensitivity that cannot be accounted for on the basis of metric or pixel-based changes alone. This NAP advantage may provide the neural basis for the greater detectability of NAP compared with MP changes in human psychophysics.

Animals↗