Questions: Spatial Attention and Posterior Parietal Cortex

Question 1

A patient with right posterior parietal damage fails to acknowledge people on their left, eats only from the right side of their plate, and marks a horizontal line's midpoint well to the right of center. What best describes this patient's deficit?

• A. Cortical blindness in the left visual field due to damage to primary visual cortex
• B. Paralysis of leftward eye movements due to damage to oculomotor control regions
• C. Hemispatial neglect: failure of spatial representation and attentional orienting toward the contralesional side, despite intact peripheral vision
• D. Visual agnosia for objects on the left side due to damage to the ventral object-recognition stream

Answer: (C)

The patient has hemispatial neglect — the critical distinction from blindness is that the primary sensory pathway may be intact; the problem is attentional and representational. Neglect patients can detect high-contrast stimuli in their neglected field under forced-choice conditions, demonstrating early visual cortex receives the input. What fails is the parietal attentional signal that brings left-side stimuli into awareness. The line-bisection error (marking right of center) is a classic neglect sign: attention is systematically biased rightward, effectively shrinking the left half. Options A and D describe primary sensory or recognition deficits, not attentional ones.

Question 2

Parietal neurons modulate spatial attention through multiplicative gain rather than additive enhancement. Compared to additive enhancement, what is the key consequence of multiplicative gain?

• A. Multiplicative gain creates a smaller increase in response overall, since multiplying by a factor less than 2 adds fewer spikes than a fixed additive amount
• B. Multiplicative gain allows neurons to respond to stimuli outside their normal selectivity, expanding their receptive field
• C. Multiplicative gain scales the entire response curve upward, amplifying signal-to-noise ratio across the full dynamic range while preserving the neuron's selectivity
• D. Multiplicative gain lowers threshold, allowing neurons to fire to stimuli that previously fell below the activation level

Answer: (C)

If a neuron fires 10, 20, 40 spikes to dim, medium, and bright stimuli, multiplicative gain (×2) produces 20, 40, 80 — preserving contrast ratios and actually increasing absolute signal-to-noise differences. Additive enhancement (+10) would produce 20, 30, 50 — compressing the contrast between stimuli at high intensities. Moreover, the neuron's selectivity (what it responds to) doesn't change, only the magnitude. This mechanism propagates attention's effects throughout the processing hierarchy without altering what information is being processed — only how faithfully it is represented.

Question 3

Left parietal damage causes more severe hemispatial neglect than right parietal damage, because the left hemisphere controls the dominant right hand and therefore receives more attentional resources.

• T. True
• F. False

Answer: False

This reverses the clinical reality. Right parietal damage causes far more severe neglect due to a hemispheric asymmetry in spatial representation: the right hemisphere contains spatial maps for both left and right space, while the left hemisphere handles predominantly right space. When the right hemisphere is damaged, both hemispheres lose coverage of left space — there is no compensatory representation remaining. When the left hemisphere is damaged, the right hemisphere's coverage of right space is intact and its partial coverage of left space persists. The dominant-hand account confuses motor lateralization (which is left-hemisphere-dominant) with spatial representation (which is right-hemisphere-dominant).

Question 4

The intraparietal sulcus encodes spatial locations simultaneously in multiple reference frames — including retinal, head-centered, and body-centered coordinates — allowing the attention spotlight to remain anchored to objects even as the eyes and head move.

• T. True
• F. False

Answer: True

Multi-frame coding is what makes parietal spatial representations useful for both perception and action. Retinal coordinates track where stimuli fall on the retina (which shifts with every eye movement); head-centered coordinates track position relative to the head; body-centered coordinates track position relative to the trunk. IPS neurons integrate retinal signals with extraretinal eye-position signals to maintain stable object representations. This allows the attention spotlight to remain locked onto an object of interest through eye movements, and to interface with motor commands for reaching (body-centered) and saccades (retinal).

Question 5

Explain why hemispatial neglect is not simply blindness in the contralesional visual field. What is the actual nature of the deficit, and what clinical evidence demonstrates the distinction?

Model answer: Hemispatial neglect is a failure of spatial representation and attentional orienting, not a sensory deficit. Primary visual pathways (retina → LGN → V1) may be entirely intact. Under forced-choice conditions, neglect patients can detect that something appeared on their neglected side — the information reaches early visual cortex. What fails is the parietal-driven attentional process that would orient awareness toward that location. Clinical evidence: neglect patients copy only the right half of drawings even when instructed to copy the whole figure; they cross out marks only on the right side of a page with uniformly distributed marks; they can sometimes describe neglected objects under strong prompting. A truly blind patient would show none of these directional asymmetries — they simply would not see on either side.

The distinction between not seeing (blindness) and not attending (neglect) is fundamental to understanding parietal function. Neglect reveals that conscious perception requires not just sensory input but an active attentional mechanism that selects spatial locations for awareness — and that mechanism is mediated by the parietal cortex.