MENTAL REPRESENATIONS

Neur 3680 Midterm I review

Mental Representations

• Mental representations can start with sensory input and progress to more abstract forms

– Local features such as colors, line orientation, brightness, motion are represented at low levels

A “labeled line”-Activity on this unit “means” that a line is present-Does the line actually have to be present?

Mental Representations

• Mental representations can start with sensory input and progress to more abstract forms

– Local features such as colors, line orientation, brightness, motion are represented at low levels

A “labeled line”-Activity on this unit “means” that a line is present-Does the line actually have to be present?

texture defined boundaries are representations arrived at by synthesizing the local texture features

the representation is embellished and extended

Mental Representations can be transformed

SAME

MIRROR-REVERSED

Mental representations

• Posner letter matching task– Same category or different?– Physically the same?

• Stroop task– interference

First Principles

• What are some ways that information might be represented by neurons?

– Magnitude might be represented by firing rate (e.g. brightness)– Presence or absence of a feature or piece of information might be represented by

whether certain neurons are active or not – the “labeled line” (e.g. color, orientation, pitch)

– Conjunctions of features might be represented by coordinated activity between two such labeled lines

– Binding of component features might be represented by synchronization of units in a network

Posner and his colleagues had participants view two letters and respond according to whether these letters were both vowels, both consonants, or one of each. Participants were fastest when viewing two physically identical letters, somewhat slower when viewing the same letter in two different fonts, and slowest in the case where two different consonants were presented. This finding shows that

a. we form multiple representations of stimuli.b. we form representations of stimuli based only on their

physical attributes.c. we form representations of stimuli based only on their

abstract category membership.d. more complex mental representations produce faster

reaction times.

Posner and his colleagues had participants view two letters and respond according to whether these letters were both vowels, both consonants, or one of each. Participants were fastest when viewing two physically identical letters, somewhat slower when viewing the same letter in two different fonts, and slowest in the case where two different consonants were presented. This finding shows that

a. we form multiple representations of stimuli.b. we form representations of stimuli based only on their

physical attributes.c. we form representations of stimuli based only on their

abstract category membership.d. more complex mental representations produce faster

reaction times.

VISION

Neur 3680 Midterm I review

Visual Pathways

• Image is focused on the retina

• Fovea is the centre of visual field– highest acuity

• Peripheral retina receives periphery of visual field– lower acuity– sensitive under low light

Retina has distinct layers…

Photoreceptors:• Rods and cones respond to

different wavelengths

Amacrine and bipolar cells perform “early” processingconverging / diverging input from receptorslateral inhibition leads to centre/surround receptive fields - first step in shaping “tuning properties” of higher-level neurons

signals converge onto ganglion cells which send action potentials to the (LGN)

• two kinds of ganglion cells: • Magnocellular and Parvocellular

RETINA

LGN

Lateral Geniculate Nucleus maintains segregation:

of M and P cells (mango and parvo)of left and right eyes

Primary visual cortex receives input from LGN

Primary cortex maintains distinct pathways – functional segregation

M and P pathways synapse in different layers

The Role of “Extrastriate” Areas

1. System is hierarchical

2. System is analytic and parallel

SUMMARY:

Dorsal stream area MT

M cells-interblobsRetina LGN V1 P cells- blobs

Ventral stream V4

SUMMARY: movement and direction

Dorsal stream area MT WHERE/HOW

M cells-interblobsRetina LGN V1 P cells- blobs

Ventral stream V4 WHAT

color

Subtract Voxel intensities during these scans… …from voxel

intensities during these scans

…etc.Time ->

Zeki et al.

The receptive fields in the primary visual cortex are ________ the receptive fields in the visual association areas.

a. smaller thanb. larger thanc. equal in size tod. less selective than

The receptive fields in the primary visual cortex are ________ the receptive fields in the visual association areas.

a. smaller thanb. larger thanc. equal in size tod. less selective than

The term ________ is used to describe the situation in which group 1 is impaired on task X and unimpaired on task Y and group 2 is impaired on task Y and unimpaired on task X. In contrast, the term ________ is used when group 1 is impaired on task X and unimpaired on task Y and group 2 is unimpaired on both tasks X and Y.

a. single dissociation / double dissociationb. double dissociation / single dissociationc. double dissociation / triple dissociationd. triple dissociation / double dissociation

The term ________ is used to describe the situation in which group 1 is impaired on task X and unimpaired on task Y and group 2 is impaired on task Y and unimpaired on task X. In contrast, the term ________ is used when group 1 is impaired on task X and unimpaired on task Y and group 2 is unimpaired on both tasks X and Y.

a. single dissociation / double dissociationb. double dissociation / single dissociationc. double dissociation / triple dissociationd. triple dissociation / double dissociation

Subtract Voxel intensities during these scans… …from voxel

intensities during these scans

…etc.Time ->

Zeki et al.

Stroke = Achromatopsia

Subtract Voxel intensities during these scans… …from voxel

intensities during these scans

…etc.Time ->

MOVING STATIONARY MOVING STATIONARY

Subtract Voxel intensities during these scans… …from voxel

intensities during these scans

…etc.Time ->

MOVING STATIONARY MOVING STATIONARY

Stroke = akinetopsia

Visual Neuron Responses• This conceptualization of the visual system was “static” - it did not take into account the

possibility that visual cells might change their response selectivity over time

– Logic went like this: if the cell is firing, its preferred line/edge must be present and…

– if the preferred line/edge is present, the cell must be firing

• We will encounter examples in which neither of these are true!

• Representing boundaries must be more complicated than simple edge detection!

•Colour•Brightness

EDGES

•Texture•Motion cues•Depth cues

The distinct modes of vision offered by feedforward and recurrent processing

Victor A.F. Lamme and Pieter R. Roelfsema

Dorsal vs. Ventral stream- spatial and object vision

Pre-attentive vs. Attentive

Conscious vs. Unconscious

Dichotomies:

The Feed-Forward Sweep

• The feed-forward sweep is the initial response of each visual area “in turn” as information is passed to it from a “lower” area

• Characteristics:– a single spike per synapse– no time for lateral connections – no time for feedback connections

The Feed-Forward Sweep

• The feed-forward sweep is the initial response of each visual area “in turn” as information is passed to it from a “lower” area

• Characteristics:– a single spike per synapse– no time for lateral connections – no time for feedback connections

Curve tracing– monkey indicates whether a particular

segment is on a particular curve– requires attention to scan the curve and

“select” all segments that belong together– that is: make a representation of the entire

curve– takes time

What is the binding problem?

QUESTIONS: