Attention Modulates Responses in the Human Lateral Geniculate Nucleus

Attention Modulates Responses in the Human Lateral Geniculate NucleusNature Neuroscience, 2002, 5(11): 1203-1210

Presented by Juan Mo

1Study Questionswhat is the structure and function of LGN?What is Functional Magnetic Resonance Imaging?What is BOLD response?In what ways that attention modulates neural activity in the human LGN?

AttentionHuman select particular aspects of a visual scene for detailed analysis and control of subsequent behavior, but ignore other aspects so completely that moments after they disappear from view we cannot report anything about them. Nat. rev. Neurosic., 2000, 1: 91-1003Ways that Selective Attention Modulates Neural Activity in the Visual SystemAttention affects processing at the first stage of cortical information processing, in the primary visual cortex.Attention not only modulates the gain on incoming visual information, but can also add a pure top-down signal that increases baseline activity in striate and extrastriate cortex.Attention can under different conditions select locations, features, objects or a combination thereof.Large regions within the fronto-parietal network, which apparently provide the source of top-down blas signals in visual areas, support a very heterogeneous set of attention tasks.4LGN Acts as the Gateway for Sensory information to Enter the Visual Cortex

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6Functional Magnetic Resonance ImagingfMRI: relies on the fact that cerebral blood flow and neuronal activation are coupled.BOLD (blood-oxygen-level dependent)7

MRI scannerSubject inside Head Coil8Relationship Between Glucose Metabolism and Blood Flow

Sokoloff (1981) used autoradiographyMeasured both glucose metabolism and blood flow39 brain regions in rat brainCorrelation r=0.95Slope m=2.69Basic Model of Relationship Between BOLD fMRI & Neuronal Activity

10Comparison of Neurophysiological/Neuroimaging Techniques: Spatial and Temporal ResolutionTemporal ResolutionSpatial ResolutionEEG/MEGElectro-physiology10 m0.1 mm1 mm1 cmPET1 ms10 ms0.1 s1 s10 s10 cmfMRIAll these methodologies have been used in both monkeys and humans.11Time Course of BOLD ResponseBOLD = Blood oxygenation-level dependent% Response Change from Background Levels

Time (seconds)Courtesy of Dr. Geoffrey BoyntonSalk Institute12Courtesy of Dr. David Heeger, New York UniversityA Demo of BOLD Response to Visual Stimuli

13Ways that Selective Attention Affects Visual Processing at the Cortex LevelAttentional enhancement: Neural responses to visual stimuli are greater when those stimuli are attended versus when they are ignored.Attentional suppression: Neural responses to ignored stimuli are attenuated depending on the load of attentional resources engaged elsewhere.Attention-related baseline increases: Directing attention to a location in the absence of visual stimulation and in anticipation of stimulus onset increases neural baseline activity.HypothesisAttentional response modulation is not confined to cortical processing, but can occur as early as the thalamic level in three ways: attentional enhancement, attentional suppression and attention-related baseline increases.Figure 1 Visual Stimuli and Experimental Design

High-contrast checkerboard stimuliLow-contrast checkerboard stimuli16

17Experiment 1Checkerboard stimuli of high- or low- contrast were presented to the left or right hemifield while subjects directed attention to the stimulus (attended condition) or away from the stimulus (unattended condition) in separate runs.Red: attended conditionHigh-contrast stimulusLow-contrast stimulusBlack: unattended condition

LGNFigure 2 Enhancement of Responses to Attended StimuliUnattended condition: subjects counted letters at fixation, which directed attention away from the stimulus.Attended condition: subjects were instructed to covertly direct attention to the checkerboard stimulus.19Experiment 2High- and low- contrast checkerboard stimuli were presented to the left or right hemifield while subjects performed either an easy or difficult attention task at fixation and ignored the peripheral checkerboard stimuli.Figure 2 Suppression of Responses to Ignored Stimuli

High-contrast stimulusLow-contrast stimulusBlack: hard task conditionBlue: easy task conditionEasy attention task: subjects counted brief, infrequent color changes of the fixation cross.Hard attention task: subjects counted letters at fixation. 21Experiment 3Subjects were cued to covertly direct attention to the periphery of the left or right visual hemifield and to expect the onset of the stimulus. After the expectation period, during which subjects attended to the periphery without receiving visual input, a high-contrast checkerboard was presented at the expected location.Figure 2 Attention-related Increases of Baseline Activity

Directed attention to the periphery of the visual hemifield in expectation of the stimulus onsetPeriods of checkerboard presentation23Figure 2 Similar Effects of Attention in Visual Cortex

24Figure 3 Attention response Modulation in the LGN and in Visual Cortical Areas

All three attentional effects increased from earlier to later processing stages along both the ventral and dorsal pathways of visual cortex.

25Figure 3 Attention response Modulation in the LGN and in Visual Cortical Areas

All three attentional effects tended to be stronger in the LGN than in striate cortex.26Experiment 4High-contrast checkerboard stimuli are presented simultaneously to both hemifields. Subjects are instructed to direct attention either to the left or right checkerboard and to detect luminance changes as in experiments 1 and 3.Figure 4 Spatial Selectivity of Attention Effects in the LGN and in Visual Cortex

AttendedUnattended28Figure 5 Eye Movement Controls

Horizontal eye position left of fixation Vertical eye position left of fixationHorizontal eye position right of fixationVertical eye position right of fixation29ConclusionsSelective attention modulates neural activity in the LGN in three ways: 1)by enhancing neural responses to attended stimuli, 2)by attenuating neural responses to ignored stimuli, 3)by increasing baseline activity in the absence of visual stimulation.The effects of attentional response are spatially specific.Eye movements are not systematically correlated with the different task conditions.

The HGN may be the first stage in the processing of visual information that is modulated by attentional signalsStudy Questionswhat is the structure and function of LGN?What is Functional Magnetic Resonance Imaging?What is BOLD response?What are the ways that attention modulates neural activity in the human LGN?