Learning generalisation and localisation: Competition for stimulus type and receptive field

Can inhibition resolve retrieval competition through the control of spreading activation?

Two experiments are reported in which the mechanisms underlying retrieval-induced forgetting for complex prose materials were investigated, using the independent probe technique pioneered by Anderson and Spellman (1995). These experiments provide additional empirical evidence in support of an inhibitory account of memory. Specifically, evidence emerged not only for the inhibition of nonpracticed items from practiced sets (i.e., first-order effects), but also for the inhibition of items from nonpracticed sets that were semantically related to practiced items in practiced sets (i.e., cross-category effects) and for items from nonpracticed sets that were semantically related to nonpracticed items in practiced sets (ie., second-order effects). These findings are considered in terms of Anderson and Spellman's model of inhibitory processing. We also outline an alternative inhibitory interpretation. Specifically, we consider how inhibition may function as a way of controlling the spread of activation and what implications this may have for the flexibility and adaptiveness of memory.

Learning viewpoint invariant perceptual representations from cluttered images

In order to perform object recognition, it is necessary to form perceptual representations that are sufficiently specific to distinguish between objects, but that are also sufficiently flexible to generalize across changes in location, rotation, and scale. A standard method for learning perceptual representations that are invariant to viewpoint is to form temporal associations across image sequences showing object transformations. However, this method requires that individual stimuli be presented in isolation and is therefore unlikely to succeed in real-world applications where multiple objects can co-occur in the visual input. This paper proposes a simple modification to the learning method that can overcome this limitation and results in more robust learning of invariant representations.

The temporal resolution of neural codes: does response latency have a unique role?

This article reviews the nature of the neural code in non-human primate cortex and assesses the potential for neurons to carry two or more signals simultaneously. Neurophysiological recordings from visual and motor systems indicate that the evidence for a role for precisely timed spikes relative to other spike times (ca. 1-10 ms resolution) is inconclusive. This indicates that the visual system does not carry a signal that identifies whether the responses were elicited when the stimulus was attended or not. Simulations show that the absence of such a signal reduces, but does not eliminate, the increased discrimination between stimuli that are attended compared with when the stimuli are unattended. The increased accuracy asymptotes with increased gain control, indicating limited benefit from increasing attention. The absence of a signal identifying the attentional state under which stimuli were viewed can produce the greatest discrimination between attended and unattended stimuli. Furthermore, the greatest reduction in discrimination errors occurs for a limited range of gain control, again indicating that attention effects are limited. By contrast to precisely timed patterns of spikes where the timing is relative to other spikes, response latency provides a fine temporal resolution signal (ca. 10 ms resolution) that carries information that is unavailable from coarse temporal response measures. Changes in response latency and changes in response magnitude can give rise to different predictions for the patterns of reaction times. The predictions are verified, and it is shown that the standard method for distinguishing executive and slave processes is only valid if the representations of interest, as evidenced by the neural code, are known. Overall, the data indicate that the signalling evident in neural signals is restricted to the spike count and the precise times of spikes relative to stimulus onset (response latency). These coding issues have implications for our understanding of cognitive models of attention and the roles of executive and slave systems.

Self-organization of shift-invariant receptive fields

This paper proposes a new learning rule by which cells with shift-invariant receptive fields are self-organized. With this learning rule, cells similar to simple and complex cells in the primary visual cortex are generated in a network. To demonstrate the new learning rule, we simulate a three-layered network that consists of an input layer (or the retina), a layer of S-cells (or simple cells), and a layer of C-cells (or complex cells). During the learning, straight lines of various orientations sweep across the input layer. Here both S- and C-cells are created through competition. Although S-cells compete depending on their instantaneous outputs, C-cells compete depending on the traces (or temporal averages) of their outputs. For the self-organization of S-cells, only winner S-cells increase their input connections in a similar way to that for the neocognitron. In other words, the winner S-cells have LTP (long term potentiation) in their input connections. For the self-organization of C-cells, however, loser C-cells decrease their input connections (LTD=long term depression), while winners increase their input connections (LTP). Here both S- and C-cells are accompanied by inhibitory cells. Modification of inhibitory connections together with excitatory connections is important for creation of C-cells as well as S-cells.