Visual field map clusters in human cortex PNAS Wandell 2005

Dec 24, 2010 08:59 PM

Brian Wandell, Alyssa Brewer and Robert Dougherty

Royal Soc  (Philos Transact)  2005

We describe the location and general properties of nine human visual field maps.

The cortical location of each map, as well as many examples of the 

eccentricity and angular representations within these maps, are shown

 in a series of images that summarize a large set of functional MRI data.

The organization and properties of these maps are compared and contrasted with descriptions by other investigators.

We hypothesize that the human visual field maps are arranged in several clusters,

each comprising a group of maps that share a common foveal representation and semicircular eccentricity map.  The spatial organization of these clusters suggests that the perceptual processing within each cluster serves related functions.

The axons in a single cubic mm of cortex extends 3-4 km in length!!!

(I calculate that total wiring is over 1 million km, all packed into 1.5 quarts)

(calculation = 1000 mm3 in a cubic cm * 1500 cc * 3-4 km per cc = over 1 million miles of wiring!!)

visual field maps preserve the spatial structure of the scene itself:  (see Kosslyn lectures)

nearby scene points are represented in the responses of nearby neurons.

Visual field maps then facilitate computations involving spatially localized comparisons

between neurons specialized for carrying information about color, motion or orientation.

Perhaps the best understood visual field map architecture is that of the retina.

The cone mosaics in the retina sample the image and form three interleaved visual field maps;

each map measures a different spectral band. Subsequent retinal wiring compares and combines

signals from these cone mosaics into a dozen different retinal ganglion cell mosaics with receptive fields that measure different spatial, chromatic and temporal image features of the image. These output neurons again comprise a set of mosaics that are arranged as interleaved visual field maps. The basic architecture of the retina then begins with the cone mosaics that comprise interleaved visual field maps. These mosaics measure different aspects of the signal.

The interleaved maps form the basis of retinal computations that compare nearby values in the image. The output is again an interleaved array of retinal ganglion cell mosaics arranged as maps.

At a scale of several centimeters, we find that human maps seem to be arranged in spatial clusters. The spatial organization of the maps themselves may also play a role in coordinating neural computations. We suspect that within each cluster neurons share common computational resources. For example, the neurons might share short-term information storage or mechanisms that coordinate neural timing (<a class="cite-reflink bibr popnode" href="#bib106" rid="bib106">Wandell et al. 2002</a>). (the paper with Bernt Girod)

On this view, information is organized at several scales. The interleaved neural mosaics within a visual field map

permit dense interconnections between units that process very similar information. Clusters of visual field maps, in turn, coordinate information sharing between maps while minimizing long-range axonal connections (<a class="cite-reflink bibr popnode" href="/pubmed/9292198" ref="reftype=pubmed&article-id=1569486&issue-id=134710&journal-id=136&FROM=Article%7CBody&TO=Entrez%7CPubMed%7CRecord&rendering-type=normal" rid="bib57">Kaas 1997b</a>).

We hypothesize that the functions carried out in clusters represent common perceptual processes, while the functions in different clusters are relatively independent, much as audition and vision are loosely coordinated.

2  OVERVIEW

a) map the visual field maps onto inflated surface maps of the brain

(that map the areas deep within a sulcus onto an artificial surface -

use shading to show the areas deep within a sulcus)

areas in a sulcus are dark and those on a gyrus are light.

b) measuring maps: to measure maps one uses traveling waves or phase-encoded methods.

Traveling wave method measures the angular directions and the eccentricity bands that excite

various cortical locations.  But you must choose stimuli that will actually stimulate the

areas of interest.

The failure to find a map is not the same as the discovery of a map.

(failure to find a map does not mean that one does not exist (if one had used the right stimuli)

c)  See figure 2... shows positions of the maps.  9 visual hemifield maps.

Each map represents the contralateral visual field.

V1 falls maily in the calcarine sulcus.

V2 and V3 maps form cortical strips surrounding V1.

Good agreement on the presence of a regions of motion selectivity on the

lateral surface of the occipital lobae near the tempor lobe.

This is region hMT+   (h prob stands for human).

The + symbol is appended to the name because, in addition to the human homologue of

maquaque MT (v5),  hMT+ is likely to contain additional cortical maps.

3)  Posterior-Medial maps  (V1 V2 V3)

The posterior-medial surface, extending from the occipital pole anterior along the calcarine sulcus, contains three hemifield maps. The primary visual cortex (V1), which receives direct input from the retino-geniculate pathway, occupies the calcarine cortex.

Two additional maps V2 and V3 occupy a stripe of cortex about 1-3 cm wide, which encircles V1. Within these maps, the cortex responds powerfully to many visual stimuli. Localized cortical damage typically results in a general loss of visual function restricted to a corresponding region within the visual field (<a class="cite-reflink bibr popnode" href="/pubmed/1884174" ref="reftype=pubmed&article-id=1569486&issue-id=134710&journal-id=136&FROM=Article%7CBody&TO=Entrez%7CPubMed%7CRecord&rendering-type=normal" rid="bib49">Horton & Hoyt 1991a</a>,<a class="cite-reflink bibr popnode" href="/pubmed/2043069" ref="reftype=pubmed&article-id=1569486&issue-id=134710&journal-id=136&FROM=Article%7CBody&TO=Entrez%7CPubMed%7CRecord&rendering-type=normal" rid="bib50">b</a>). Although damage causes severe visual dysfunction, in some neurological cases residual visual capabilities remain (<a class="cite-reflink bibr popnode" href="/pubmed/15204125" ref="reftype=pubmed&article-id=1569486&issue-id=134710&journal-id=136&FROM=Article%7CBody&TO=Entrez%7CPubMed%7CRecord&rendering-type=normal" rid="bib18">Cowey 2004</a>; <a class="cite-reflink bibr popnode" href="/pubmed/14650852" ref="reftype=pubmed&article-id=1569486&issue-id=134710&journal-id=136&FROM=Article%7CBody&TO=Entrez%7CPubMed%7CRecord&rendering-type=normal" rid="bib107">Weiskrantz 2004</a>).

each of V1, V2, and V3 have separate maps.  organization of these is the same in humans and macaques.

These three maps are aligned (co-registered ) as in fig 3B.

(one could merge these three cell mosaics into a single map).

The cortical surface area repres. the central visual field far exceeds that devoted to the peripheral field.

(surface area of V1 and V2 varies by a factor of two across individuals and is strongly correl with the size of the

optic neruve and lateral genic.)

Human V1 is 4000 mm2 =  40 cm2 =  (my calc, abt  6.5 by 6 cm)

(compares to macaque at 2000 mm2)

4)  VENTRAL MAPS  hV4 and VO-1

This region of cortex responds powerfully and selectively during object recog taks as S views

targets including faces, objects, text or even simple colored patterns.  (Kanwisher 97)

h = human  to clarify that its somewhat difft then the V4 in other species.

Another map is VO-1  VO stands for ventral occipital  (it is anterior to hV4 on the ventral surface)

there may be other VO maps also.  There is a large emphasis on the FOVEAL REPRES on the ventral

surface ... stimuli in the central 5 degrees evoke powerful responses in this region.

5)  DORSAL MAPS  V3A  V3B and V7

go from the poseterior portion of the intrapareital sulcus forward and contain several maps.

They respond powerfully to MOTION and DEPTH PERCEPTION.

6) LATERAL MAPS  hMT+ and LOC

The anterior portion of the lateral occipital  (LO) lobe extending into the super temporal sulcus,

responds powerfully to motion and attention.  (damage causes disruption to motion perception)

LOC = lateral occip complex  responds powerfully in object recog tasks.

The surface area of the LO complex is commensurate  with that of V1.

DISCUSSION (7)

visual field maps coordinate signals across multiple levels of spatial resolution.

We speculate that these map clusters share resources such as circuitry that coordinates the timing

of neural signals or tempor stores local calculations  (short-term memory)  (see the 2002 Wandell paper with Gerod)

They discuss differences btwn what they find and studies by Malach and Grill-Spector.

They question whether tghe macaque should serve as the gold standard for human visual areas.

THERE IS NO CONCENSUS ON MACAQUE VISUAL AREAS!!  (van essen 03).

V1 V2 and MT are well established but everything else is not in concensus.

There is a 25 million year evol. divergence btwn macaques and humans... therefore, signif diffs.

8 CONCLUSION

Nine human visual hemifield maps present in each hemisphere.

Clusters are organized by function.

Visual field map clusters in human cortex PNAS Wandell 2005

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