Bernal S D_2010.pdf - University of Plymouth

2.1. OBJECT REC(X}NIT!ON
tivity, stem from the original Hubel and Wiescl proposal (Hubel and Wiesel 1965), and have
been supported by posterior physiological findings. Neurons in area V4 in the primate {Gawne
and Martin 2002} and complex cells in the cat visual cortex (Lampl et al. 20(>4) have both been
found to show responses that can be predicted relatively well by the nave operation. In the latter
study, when optimal and non-optimal bars were presented simultaneously, the response of the
complex cells closely resembled the response when the optimal stimulus was presented alone.
A recent study (Masquelier et al. 2007) demimstrales the plausibility of this mechanism, by
learning complex cell invariance from natural videos. For the selectivity operation, a normal­
ized dot product operation followed by a sigmoid function has been suggested as a biologically
plausible implementation (Serre et al. 2005a. 2(K)7c).
Although HMAX is a relatively abstract model, several attempts have been made to show its
validity al a lower level of description. The max operation, which achieves invariance. has
been shown to be implementahlc by different biologically plausible circuits, the most likely
being the cortical microcircuits consisting of lateral and recurrent inhibition (Yu el al. 2002).
Interestingly, a similar study (Kouh and Poggio 200X) extended the previous results showing
how the two distinct neural operations, selectivity and invariance, were approximated by the
same canonical circuit, involving divisive normahzation and nonlinear operations. The circuit
was based on neurophysiological dala suggesting Ihc existence of a basic cortical structure
similar wilhin and across different functional areas. At the biophysical level of description,
Knoblich et al. (2007) propo,sed a detailed model that could approximate the HMAX operations,
based on standard spiking and synaptic mechanisms found in the visual and barrel cortices.
Their model was shown to implement both the invariance and tuning operations, satisfying
the timing and accuracy constraints required to perform object n^ognition in a biologically
plausible manner.
Taken as a whole the HMAX model provides useful insights into how ihe selectivity and invari­
ance properties observed along the ventral path can be gradually built. It is grounded on widely
accepted neurophysiological principles, such as a hierarchical increase in receptive field size
and complexity. The model provides a relatively good fit to VI cells' tuning paraineters and
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