"(on the nature of the attentive process)…two physiological processes: 1) The accommodation or adjustment of the sensory organs. 2) The anticipatory preparation from within of the ideational centres concerned with the object to which the attention is paid."

-William James, from The Principles of Psychology (pg 434)

Humans are endowed with sensory organs that allow for the acquisition of environmental signals in the form of light, sound, chemical smell, chemical taste, and mechanical pressure. It is through processing of information from these five channels that the brain is able to create representations of the outside world, store memories of combinations of these representations, and generate motor outputs that are consistent with the immediate surroundings and constraints applied to the organism.

It is well known to psychologists that the brain is not merely a passive medium for the imprinting of sensory images. Aspects of the environment must be attended to or they will be filtered out by the brain. This filter is active in the sense that prior histories of sensory inputs and brain states change over time, resulting in a plasticity of sensory acquisition. In order to address these ideas of attentional filtering and representation of sensory objects, this project will explore the active electrosensory system of the pulse-emitting weakly electric fish, Gnathonemus petersii.

While there is wide variation in the electric field discharge properties of different families within each order of weakly electric fish, within-family waveforms are relatively homogenous (Heiligenberg, 1990). Fish within the order Osteoglossiformes and family Mormyridae, such as Gnathonemus petersii, emit a discontinuous pulse waveform. Many families within the order Gymnotiformes, such as Eigenmania, emit continuous waveforms that can be viewed as smooth sine waves with various frequency components (Heiligenberg, 1990). Waveforms are modulated under certain behavioral conditions, as in the accentuated sexual dimorphism of discharge frequency during mating season, increases in frequency during aggressive encounters and engagement of novel stimuli, and frequency shifts to avoid signal jamming by conspecifics (Heiligenberg, 1990).

The weakly electric fish locates objects by creating neural representations based on the distortions in its own electric field caused by the conductive and resistive properties of those objects. The latency-to-fire of electroreceptors, which transduce electrical signals, is proportional to the intensity of the current inward across the skin; a large inward current will cause a receptor to generate an action potential in its afferent fiber with a shorter delay than will a current of smaller magnitude (Bell, 1989).

This initial latency-to-fire representation of an object in the fish’s environment is then transformed, at the first central relay, into a burst duration code representation at the granular cell layer of the fish’s electrosensory lateral line lobe (ELL). A copy of the command that causes the electric organ to discharge (corollary discharge) provides coincident input to the ELL that enhances afferent inputs (incoming sensory signals) from the cutaneous electroreceptors that arrive immediately after the motor command and which could have been evoked by the fish’s own discharge (Bell, 1989). This AND gating (via coincidence detection) of sensory input to the system responsible for active electrolocation enhances reafferent signals, that is, signal distortion of the fish’s own "motor" output (figure 1).

As an analogy, the electric organ discharge serves as a strobe light which "illuminates" the vicinity of the electric fish. Reafferent signals are only sensible if they occur when this strobe light is "on," and so the active electrolocation system is specially constructed to enhance sensory input which is time-locked to the electric organ discharge, functionally filtering out input when the strobe is off. This active process allows the fish to filter out electrical noise, produced by conspecifics and other external field sources such as lightning, so that it can attain precise representations of the spatial properties of its environment. The AND gate also serves a second purpose, the decoding of stimulus intensity arriving from the primary afferent as a latency code; the coincidence detector serves the dual function of enhancing inputs that arrive within a defined temporal window and extracting information about stimulus intensity from the temporal location of the inputs within that window.

Knowledge of the morphologies, ion channels, synaptic inputs, and voltage responses of the neurons involved in this spatial object representation allows for the creation of a computer model. This model will facilitate understanding of the active neural processes involved in creating a spatial representation of objects at the ELL granular cell layer of Gnathonemus petersii. Specifically, the model will address transformation of the latency-to-fire code for stimulus intensity into a burst duration code in a single granular cell. I will outline the physiology, behavior, and sensory systems of weakly electric fish before addressing the model in detail.

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