Questions: Sensory Transduction and Neural Coding

Question 1

A subject holds a 1 kg weight continuously for 10 minutes. The weight initially feels very heavy, but the sensation fades. When a researcher quietly adds 50 grams, the subject immediately notices the change. Which aspect of sensory coding best explains this pattern?

• A. Rate coding failure — the sensory neurons have exhausted their supply of neurotransmitter
• B. Adaptation reduces the neural response to the constant weight, freeing the system's sensitivity to detect the new change in stimulus
• C. Temporal coding shifts so that spike timing encodes the new weight more precisely
• D. The thalamus actively suppresses the constant signal through inhibitory gating

Answer: (B)

Rapidly adapting mechanoreceptors (like Meissner's corpuscles) respond strongly to stimulus onset and then fall silent during sustained contact, while slowly adapting receptors (Merkel discs) maintain a tonic but diminished signal. This adaptation is not a failure — it is a feature: by reducing the neural 'noise' of constant stimulation, the system becomes exquisitely sensitive to changes. The 50-gram addition is detected precisely because the adapted baseline makes the change stand out. This illustrates why adaptation is understood as enhancing change detection rather than degrading stimulus representation.

Question 2

How does rate coding differ from temporal coding as a strategy for representing sensory information in neural signals?

• A. Rate coding uses the total number of spikes over a lifetime; temporal coding uses spikes within a single trial
• B. Rate coding encodes stimulus intensity through firing frequency; temporal coding carries information in the precise timing of individual spikes relative to the stimulus
• C. Rate coding applies only to the somatosensory system; temporal coding applies only to the auditory system
• D. Rate coding is used by peripheral receptors; temporal coding is used exclusively by cortical neurons

Answer: (B)

In rate coding, a stronger stimulus produces a higher firing rate — the neuron's 'message' is how fast it fires, averaged over a time window. In temporal coding, the specific timing of each spike carries information independent of rate. In the auditory system, neurons can phase-lock their spikes to the periodicity of a sound wave at low frequencies, encoding the wave's frequency directly in spike timing. Both strategies are used simultaneously in real neural circuits, and population coding distributes the representation further across neurons with different tuning preferences. No single strategy is sufficient for all sensory dimensions.

Question 3

Sensory adaptation represents a failure of the nervous system to maintain accurate representation of a sustained stimulus.

• T. True
• F. False

Answer: False

Adaptation is an adaptive feature, not a failure. By reducing responsiveness to unchanging stimuli, the sensory system frees up dynamic range and computational resources for detecting changes — which are generally more behaviorally relevant than static conditions. An unvarying stimulus (a constant smell, constant pressure from clothing) is typically already known and acted upon; what matters is whether something changes. Rapidly adapting receptors signal stimulus onset and offset precisely, and slowly adapting receptors maintain a reduced tonic signal for sustained contact. Together they provide both event detection and background intensity information.

Question 4

Sensory transduction converts physical energy into a graded receptor potential before action potentials are generated, even for the fastest sensory pathways.

• T. True
• F. False

Answer: True

In every sensory modality, the conversion of physical energy (light, sound, pressure, chemicals) produces a graded receptor potential — a continuous analog voltage change in the receptor cell that is proportional to stimulus intensity. This receptor potential is not itself an action potential. If it exceeds threshold, it triggers the all-or-nothing action potentials that propagate along the sensory nerve. This two-stage conversion (analog receptor potential → digital action potential train) is universal: photoreceptors produce graded potentials that drive bipolar cells; hair cells produce graded potentials that synapse onto auditory nerve fibers; mechanoreceptors produce graded potentials directly in sensory neuron terminals.

Question 5

Explain why sensory adaptation is considered a feature rather than a flaw of the sensory system. What would sensory experience be like if rapidly adapting receptors did not exist?

Model answer: Adaptation is a feature because it allocates neural resources to what matters most: change. Constant stimuli, once registered and accounted for, carry little new information. By reducing the response to sustained inputs, the system maintains high sensitivity to changes — which signal behaviorally relevant events like new threats, opportunities, or environmental shifts. Without rapidly adapting receptors, every constant contact — clothing against skin, ambient odors, background sounds — would produce continuous, maximal neural activity. This would overwhelm the system's dynamic range and make it difficult to detect novel events against the noise of ordinary constant stimulation.

The evolutionary logic is detection of change over absolute state representation. The just-noticeable-difference (Weber's law) and signal detection theory both reflect this: sensory systems are not designed to report absolute stimulus intensity with precision but to detect relative changes. Rapidly adapting receptors are the implementation of change detection at the receptor level. Slowly adapting receptors provide the tonic background against which changes are measured. The two types are complementary, not redundant — together they provide a richer representation than either alone.