C2006/F2402 -- 2005 --Outline of Lecture #19 – Electrical Communication #3

(c) Stuart Firestein, Columbia University, New York, NY. Last update 04/11/2005 02:02 PM .  

Notes by Chris Kelly


Basic Ideas


Features common to all sensory systems


Features of stimuli

o Modality

§         The law of specific energies states that specialized receptors are attuned to a certain kind of energy. This is generally true: photoreceptors detect electromagnetic energy, whereas olfactory receptors are sensitive to chemicals (smells). Remember, however, that under extreme conditions, a certain stimulus can cause any receptor to respond. A punch in the eye results in a flash of light, even though it is a mechanical, and not electromagnetic, energy.

§         There are four classes of receptors in humans: mechanical, chemical, electromagnetic, and thermal.

·        Mechanical receptors are triggered by sheer physical activity. For example, touch receptors in your skin respond to physical (somatosensory) input because that input is sufficient to cause protein conformation change and, thus, depolarization. (By pushing on your skin, you are literally opening channels.) The auditory system has a complex mechanical arrangement for processing sound stimuli. See book for details on the cochlea.

·        Chemical receptors respond to molecular ligands. The olfactory and gustatory systems use chemical receptors.

·        Electromagnetic receptors respond to light energy. Rods and cones are examples, since they respond to photons.

·        Thermal receptors respond to changes in temperature.

§         Receptors are attuned to a particular modality, but they are also more finely attuned to particular qualities within that modality. For example,  all cones respond to light, but “blue” cones give maximal responses to wavelengths of 420 nm, whereas “red” cones are tuned to wavelengths of 560 nm. A tuning curve for a particular receptor plots the average response against a varied parameter; for example, one could plot how much response a particular photoreceptor gives when subjected to photons with different wavelengths. These plots allow one to determine the stimulus parameters to which that receptor gives the greatest response (and to which it is, hence, maximally tuned.)

§         Once the signal has passed beyond the initial receptor, how is its modality coded? In other words, we know that by occurring in a photoreceptor cell, an action potential must be encoding a visual stimulus –– but how do you specify the modality once the signal has moved beyond the initial receptor cell?

·        Labeled line networks.

o       An electromagnetic receptor projects to a unique network of neurons, so if those particular neurons are firing, it must be in response to an electromagnetic input. Hence the labeled line –– a designated network.

o Amplitude

§         Amplitude is typically described with dose-response curves. Along the x-axis is the intensity of the stimulus, often on a logarithmic scale. On the y-axis is the percentage of trials in which a subject can accurately detect the stimulus at that intensity. The plot usually looks sigmoidal, and the intensity for which one has a 50% accuracy rate is deemed the threshold intensity –– the minimum intensity the stimulus must have in order to be detected.

§         One problem with self-reported perception is that current experiences are modulated by previous ones. For example, if you closed your eyes and were given a one-pound weight, you would reliably report when you had received the weight. If you were first saddled with a fifty-pound weight, it would be hard to tell when the one pound weight had been added, even though its magnitude had not changed.

·        The Weber-Fechner law gives the just noticeable difference (JND), the minimum possible change in stimulus intensity that can be detected.

§         On the neural scale, how is amplitude encoded? Action potential frequency and population frequency. The former corresponds to how often a single neuron fires, the latter to how many neurons related to that stimulus are firing.

o       Location

§         Determining the location of a stimulus is very important to an organism vying to survive. It is thus a fast and robust mechanism in several of the sensory systems. There are a few methods for encoding location.

§         (1) In many systems, each neuron has a receptive field, a region of the environment to which it is sensitive. If a stimulus appears in a neuron’s receptive field, that neuron will fire. One can thus know where a particular object is based on which neurons are firing.

·        Note that the resolution of this system varies from location to location, based on the density of receptor cells: one can reliably report feeling two adjacent but distinct somatosensory inputs when those inputs are on the finger, but not when they are on the back. On your back, they just feel like a single input. The somatosensory receptors are thus denser in the fingers than on the back.

·        Also, as networks converge, resolution goes down. If ten rods, each with a discrete receptive field, all synapse onto the same bipolar cell (the next neuron in the visual network), the receptive field of that bipolar cell is equal to the sum of the fields for the rods. Resolution has thus decreased, as the field input has been reduced to a single averaged unit, instead of ten unique ones.

§         (2) Another mechanism for locating stimuli is based on comparing inputs to the two main sensors (the two eyes, two ears, etc.).

·        In the visual system, we can calculate depth by comparing the differences between the images on the right and left eyes.

·        In the auditory system, we can calculate location based on differences in signal arrival times for each ear, as well as differences in signal intensities. For example, if a sound comes from the left field, it will reach the left ear before the right ear. Also, the signal will be louder at the left ear than at the right, because of interference from the skull. These differences can be analyzed to determine location.

§         Finally, with smell, we (3) determine location based on gradients. We smell something and then move in some direction. If the smell has gotten more intense, we’re going toward its source. This is a warmer/colder location mechanism.

o       Duration

§         Some cells fire as long as the stimulus is present. Some cells briefly fire as soon as the stimulus goes on, then stop, and then briefly fire when the stimulus goes off. Still other cells fire continuously in response to the stimulus, though the magnitude of the response decreases over time (within the single stimulus presentation).

§         Let’s look closer at the last phenomenon: this is adaptation. In some sensory systems, like smell, your perception of a stimulus weakens over time. Why? There are biochemical reasons, but the effect can happen on the channel, neuronal, or cortical level. There are different terms for each.

·        Desensitization is what channels do.

·        Adaptation is what cells do.

·        Habituation is what cortex does.