Temporal Lobe

Abstract

The temporal lobe is one of the four lobes of the cerebral cortex, lying below the lateral sulcus on the side of each hemisphere. It houses the primary auditory cortex on the superior temporal gyrus, where sound is mapped by frequency; the ventral visual stream on its inferior surface, which recognises objects and faces; and, on its medial wall, the hippocampus and surrounding cortices that form new lasting memories. Its anterior pole holds a hub for conceptual knowledge, and its left side supports the comprehension of language. The lobe entered cognitive science through the patient H.M., whose bilateral medial temporal resection abolished the ability to form new declarative memories. This article traces the lobe from its auditory map through the ventral stream for objects and faces to the medial temporal memory system and the anterior semantic hub.

Keywords: temporal lobe, auditory cortex, fusiform face area, medial temporal lobe, declarative memory

The temporal lobe occupies the lower side of each cerebral hemisphere, tucked beneath the lateral sulcus and in front of the occipital lobe. It is the most functionally heterogeneous of the four lobes: a single sheet of cortex that hears, that recognises what the eyes see, that comprehends speech, and that binds experience into memory. These roles are not scattered at random. Sound enters at the top, on the superior temporal gyrus; the recognition of objects and faces runs along the bottom, the terminus of the ventral visual stream; and the machinery for forming new memories sits deepest, on the medial wall, where the hippocampus lies. This article moves through those territories in turn and closes with the amnesia, agnosia and semantic dementia that temporal-lobe damage produces.

## What the Temporal Lobe Is

The temporal lobe is one of the four lobes of each cerebral hemisphere, bounded above by the lateral (Sylvian) sulcus that separates it from the frontal and parietal lobes, and merging behind into the occipital lobe. On its lateral surface three gyri run front to back — the superior, middle and inferior temporal gyri — while its inferior and medial surfaces carry the fusiform gyrus and, folded onto the medial wall, the hippocampus and the parahippocampal gyrus. Unlike the occipital lobe, which is almost single-mindedly visual, the temporal lobe serves several senses and several kinds of cognition at once, its functions laid out by surface: audition on top, vision below, and memory within.

The lobe's involvement in experience was shown dramatically by Wilder Penfield, who during surgery for epilepsy stimulated the exposed temporal cortex of conscious patients and found that a small current could evoke not just crude sensations but full experiential responses — fragments of remembered music, scenes and voices, and dreamy states of familiarity (#ref-penfield-perot-1963). These experiential responses, elicited from the superior and lateral temporal cortex, were among the first direct evidence that the temporal lobe participates in the storage and replay of perceptual experience, not merely in the reception of sound.

Figure 1

The temporal lobe on the lateral surface, below the lateral sulcus, with the superior temporal gyrus and the approximate site of the auditory cortex marked.

## Types of Temporal Lobe

In the Medical Subject Headings (MeSH) vocabulary the temporal lobe is filed under the cerebral cortex, and it has four narrower descriptors: the auditory cortex, the perirhinal cortex, the uncinate fasciculus and the Wernicke area. MeSH is an indexing classification, not a functional parcellation — these four children are the structures NLM indexes beneath the temporal heading, and they are a mixed set, spanning a sensory cortex, a memory-related cortex, a white-matter tract and a language region rather than a single anatomical scheme. The subdivisions that matter most for cognition (the medial temporal lobe memory system, the inferotemporal object-recognition cortex, and the superior temporal speech regions) are orthogonal to this indexing hierarchy and are introduced in the sections that follow rather than in the table below. None of the four children is yet a separate article on this site, so none is linked here.

## Auditory Cortex and the Sound-to-Meaning Stream

The top of the temporal lobe is where hearing becomes cortical. The primary auditory cortex, on Heschl's gyrus within the superior temporal gyrus, receives the output of the auditory pathway and carries a tonotopic map: neighbouring patches of cortex respond to neighbouring frequencies, so the spectrum of sound is laid out in an orderly strip, much as the body surface is laid out on the somatosensory cortex. Surrounding the primary field, the planum temporale acts as a computational hub that segregates and matches the complex spectrotemporal patterns of speech, music and environmental sound, routing them onward for interpretation (#ref-griffiths-warren-2002). The site is the cortical foundation of auditory perception.

Beyond the primary map, auditory cortex is organised into streams. Recording in the non-human primate, Rauschecker and Tian found evidence for a division of labour analogous to the visual system: an anterior stream specialised for identifying what a sound is and a posterior stream more concerned with where it comes from (#ref-rauschecker-tian-2000). In the human brain this scheme was extended to language by Hickok and Poeppel, whose dual-stream model of speech processing posits a ventral stream, running into the temporal lobe, that maps sound onto meaning, and a left-dominant dorsal stream that maps sound onto articulation (#ref-hickok-poeppel-2007). The ventral, sound-to-meaning route is bilateral and relatively robust; the classic comprehension deficit of Wernicke aphasia follows damage to the posterior superior temporal region, and the mapping of speech sound onto lexical meaning is the temporal lobe's contribution to speech perception.

## The Ventral Stream: Objects and Faces

The underside of the temporal lobe is where seeing becomes recognising. Mishkin, Ungerleider and Macko drew the influential distinction between two cortical visual pathways: a dorsal occipito-parietal stream for spatial vision and a ventral occipito-temporal stream for object vision — the what pathway that identifies what is seen (#ref-mishkin-ungerleider-macko-1983). The ventral stream terminates in the inferotemporal cortex, where Tanaka showed that individual neurons respond selectively to moderately complex visual features, and that cells with similar preferences cluster into columns, giving the region a systematic organisation for object form (#ref-tanaka-1996). This ventral-temporal territory is the seat of visual object recognition.

Within this expanse are patches specialised for particular categories. Kanwisher, McDermott and Chun identified a region of the fusiform gyrus, the fusiform face area, that responds far more strongly to faces than to other objects (#ref-kanwisher-1997), and Grill-Spector and Weiner later described the broader functional architecture of the ventral temporal cortex as a mosaic of category-selective regions for faces, bodies, scenes and words, arranged by consistent anatomical landmarks (#ref-grill-spector-weiner-2014). How face-selective cells actually encode identity was resolved by Chang and Tsao, who found that neurons in the macaque face patches represent a face as a linear combination of a small number of shape and appearance dimensions — a face space — so that a face can be reconstructed from the firing of about two hundred cells (#ref-chang-tsao-2017). This account of the neural code for identity is the mechanistic core of face perception.

## The Medial Temporal Lobe and Memory

The deepest and most famous role of the temporal lobe lies on its medial wall. In 1953 the patient H.M. underwent a bilateral resection of the medial temporal lobe to relieve epilepsy, and Scoville and Milner reported the consequence: a profound and lasting inability to form new memories, with older memories and general intelligence preserved (#ref-scoville-milner-1957). H.M. could hold a conversation and improve at motor skills, yet could not remember having had the conversation or done the task. Decades of subsequent study, reviewed by Corkin, refined the lesion and the deficit, showing that his amnesia was selective for the conscious recollection of facts and events (#ref-corkin-2002). The syndrome is the defining case of amnesia.

From this and converging animal work, Squire and Zola-Morgan defined the medial temporal lobe memory system: the hippocampus together with the adjacent entorhinal, perirhinal and parahippocampal cortices, working as an organised system to establish new declarative memories (#ref-squire-zola-morgan-1991). The structures are needed to form and consolidate memory but not to hold it permanently; over time, memories become independent of the hippocampus, a process central to memory consolidation. Squire, Stark and Clark set out the anatomy and function of this system in detail (#ref-squire-stark-clark-2004), and Squire and Wixted traced how the study of human memory advanced in the decades after H.M., establishing the distinction between declarative and non-declarative memory and the graded contributions of the medial temporal structures (#ref-squire-wixted-2011). Recording from single neurons in the human medial temporal lobe, Quian Quiroga and colleagues found cells that respond to a particular person or concept across strikingly different images and even the written name — an invariant, abstract representation (#ref-quiroga-2005), and Rutishauser has shown how such single-neuron recordings can adjudicate between competing models of how declarative memory is formed and retrieved (#ref-rutishauser-2019). This medial system underlies both episodic memory and the laying down of long-term memory.

## Semantic Memory and the Anterior Temporal Lobe

The front of the temporal lobe holds knowledge of a different kind: not the record of specific events but the store of general concepts. Hodges and colleagues characterised semantic dementia, a progressive fluent aphasia in which atrophy of the anterior temporal lobes erodes conceptual knowledge — patients lose the meanings of words and objects while speech remains fluent and episodic memory relatively spared (#ref-hodges-1992). This double dissociation, semantic loss with intact event memory, mirrors the reverse pattern in medial temporal amnesia and shows that the temporal lobe holds at least two distinct memory functions in different territory. Lambon Ralph and colleagues built these findings into the hub-and-spoke account of semantic memory: modality-specific spokes across the brain feed a single transmodal hub in the anterior temporal lobe, where they are combined into coherent, generalisable concepts (#ref-lambon-ralph-2017). The anterior temporal pole thus complements the medial system: one stores what things mean, the other records what happened.

## Worked Example

The tonotopic map of the auditory cortex can be described with a simple model that the first demonstration above implements. Because the map is logarithmic in frequency, the cortical place of a tone is proportional to the logarithm of its frequency, not to the frequency itself. Let the audible range run from a lowest frequency f_min = 20 Hz to a highest f_max = 20000 Hz, and let the relative position x along the tonotopic axis run from 0 at the low-frequency end to 1 at the high-frequency end. Then the place of a tone of frequency f is:

x = log(f / f_min) / log(f_max / f_min)

The denominator is the full span of the map. Here f_max / f_min = 1000, and log₂(1000) = 9.97, so the audible range covers about 9.97 octaves.

Take a tone of 1000 Hz. Its distance above the floor is log₂(1000 / 20) = log₂(50) = 5.64 octaves, so x = 5.64 / 9.97 = 0.566 — the tone sits about 57 per cent of the way along the map. A tone of 100 Hz gives log₂(5) = 2.32 octaves and x = 2.32 / 9.97 = 0.233, about 23 per cent along. The tenfold step from 100 Hz to 1000 Hz therefore shifts the cortical place by 0.566 − 0.233 = 0.333, one third of the whole map, because a factor of ten is one third of the map's three decades.

The revealing consequence is that every octave — every doubling of frequency — occupies the same cortical distance, log₂(2) / 9.97 = 1 / 9.97 = 0.100, about a tenth of the map, wherever it falls. The octave from 500 Hz to 1000 Hz and the octave from 5000 Hz to 10000 Hz take up equal stretches of cortex, though the second spans 5000 Hz of frequency and the first only 500 Hz. This equal spacing of octaves is the signature of a logarithmic map, and the first demonstration computes x from the same formula, so its readout agrees with the arithmetic here.

## Discussion

The temporal lobe is the clearest case in the cortex of function organised by surface. Its three great roles occupy three faces of the same lobe: audition on the superior gyrus, object and face recognition along the inferior surface, and memory on the medial wall, with word meaning gathered at the anterior pole. That segregation is what makes the lobe's clinical syndromes so sharply dissociable. A medial lesion produces amnesia without agnosia, as in H.M., who could still recognise what he saw but could not remember seeing it; a ventral lesion produces agnosia or prosopagnosia without amnesia; and anterior atrophy produces the loss of concepts with events and perception intact. Few regions of the brain offer so legible a mapping between where the damage falls and what is lost.

Yet the lobe is also a study in integration. The same medial structures that record an episode draw their content from the auditory and visual cortices that surround them, and the anterior semantic hub binds together information distributed across the whole brain. Penfield's experiential responses — evoked from the lateral temporal surface but reaching into remembered scenes and music — hint at how tightly perception and memory are interwoven here. The modern picture is less a set of separate organs sharing a lobe than a gradient: from the sensory maps at the periphery, through increasingly abstract representations of objects and sounds, to the multimodal codes of the medial memory system and the anterior conceptual hub.

## Current Directions

Two lines of work are reshaping the account of temporal-lobe function. The first is the reading of neural codes at single-cell resolution. Chang and Tsao's demonstration that face identity is a linear combination of a few dozen dimensions turned face recognition from a black box into an equation that can be inverted to reconstruct the seen face (#ref-chang-tsao-2017), and human single-neuron recordings are now used to test formal models of how the medial temporal lobe forms and retrieves declarative memories rather than merely to describe its cells (#ref-rutishauser-2019). The second is the biology of the memory trace itself: Josselyn and Tonegawa have synthesised the evidence that a memory is stored in a sparse ensemble of neurons, an engram, whose reactivation recalls the memory and whose artificial stimulation can drive recall or even implant a false memory (#ref-josselyn-tonegawa-2020). In parallel, the hub-and-spoke model of semantic memory continues to be refined as imaging and computational work localise the anterior temporal hub and specify how it abstracts across modalities (#ref-lambon-ralph-2017). Together these directions are converting the temporal lobe from a map of where functions sit into an account of the codes and mechanisms by which it hears, recognises and remembers.

## Common Misconceptions

The temporal lobe is just the hearing lobe.

Hearing occupies only its superior surface. The inferior surface recognises objects and faces, the medial wall forms new memories, and the anterior pole stores word meaning — several distinct functions in one lobe (#ref-squire-stark-clark-2004).

Memories are stored permanently in the hippocampus.

The medial temporal lobe is needed to form and consolidate declarative memories, but over time they become independent of it; H.M. retained memories from well before his surgery (#ref-squire-wixted-2011).

The fusiform face area is where faces are stored.

It is a processing region selectively engaged by faces, not a gallery of stored faces. Identity is carried by a population code across many neurons, each tuned to a dimension of a face space (#ref-chang-tsao-2017).

## Glossary

Amnesia.

A loss of memory; the anterograde form, an inability to form new declarative memories, follows bilateral medial temporal lobe damage.

Auditory cortex.

The cortex of the superior temporal gyrus that first receives auditory input, centred on Heschl's gyrus and organised tonotopically.

Declarative memory.

Memory for facts and events that can be consciously recollected, dependent on the medial temporal lobe; contrasted with non-declarative memory.

Engram.

The physical memory trace, understood as a sparse ensemble of neurons whose reactivation reinstates the stored memory.

Entorhinal cortex.

Medial temporal cortex that is the main gateway of cortical information into and out of the hippocampus.

Fusiform face area.

A region of the fusiform gyrus on the ventral temporal surface that responds far more strongly to faces than to other objects.

Heschl's gyrus.

The transverse gyrus of the superior temporal plane that carries the primary auditory cortex.

Hippocampus.

A curved structure on the medial temporal wall, central to forming new declarative memories and to spatial memory.

Inferotemporal cortex.

The cortex of the inferior temporal surface at the end of the ventral visual stream, where neurons code complex object form.

Medial temporal lobe.

The hippocampus and the adjacent entorhinal, perirhinal and parahippocampal cortices, working as a system for declarative memory.

Planum temporale.

The cortical surface behind Heschl's gyrus that acts as a computational hub for segregating complex sounds, including speech.

Prosopagnosia.

An impairment in recognising faces, associated with damage to the fusiform face region of the ventral temporal cortex.

Semantic dementia.

A progressive loss of conceptual knowledge from atrophy of the anterior temporal lobes, with speech fluent and event memory spared.

Superior temporal gyrus.

The uppermost gyrus of the temporal lobe, carrying the auditory cortex and, posteriorly, regions for speech comprehension.

Tonotopy.

The orderly mapping of sound frequency onto cortex, so that neighbouring cortical sites respond to neighbouring frequencies.

Ventral stream.

The occipito-temporal visual pathway that identifies objects and faces; the what pathway, contrasted with the dorsal where pathway.

Wernicke area.

A posterior superior temporal region of the language-dominant hemisphere associated with the comprehension of speech.

## Key Researchers

Gregory Hickok. With David Poeppel, formulated the dual-stream model of speech processing: a bilateral ventral stream in the temporal lobe mapping sound to meaning and a left-dominant dorsal stream mapping sound to articulation. ORCID - UC Irvine faculty

Nancy Kanwisher. Co-discovered the fusiform face area, a region of the ventral temporal cortex selectively responsive to faces, and mapped a mosaic of category-selective regions for bodies, scenes and words. ORCID - Wikipedia

Brenda Milner (born 1918). Studied the patient H.M. after his bilateral medial temporal resection and showed that intact motor-skill learning coexisted with a dense inability to form new declarative memories, dissociating memory systems and founding the cognitive neuroscience of memory. Wikipedia - McGill faculty

Karalyn Patterson. Characterised semantic dementia — the progressive loss of conceptual knowledge from anterior temporal-lobe atrophy — and advanced the hub-and-spoke account of semantic memory. Wikipedia - MRC CBU faculty

Wilder Penfield (1891–1976). Electrically stimulated the exposed temporal lobe of conscious epilepsy patients and evoked vivid experiential responses — memories, music and dreamy states — mapping temporal-lobe contributions to perception and recollection. Wikipedia

Larry R. Squire (born 1941). Defined the medial temporal lobe memory system — hippocampus plus entorhinal, perirhinal and parahippocampal cortices — and the declarative/non-declarative distinction, integrating human amnesia with animal models. Wikipedia - UC San Diego faculty

Doris Y. Tsao. Mapped the macaque face-patch system in the inferotemporal cortex and cracked its neural code, showing that face cells encode faces as linear projections onto a small set of axes. ORCID - Wikipedia

## Frequently Asked Questions

What does the temporal lobe do?
It carries out several functions at once. Its top surface hears, holding the primary auditory cortex; its bottom surface recognises objects and faces as the end of the ventral visual stream; its medial wall forms new memories through the hippocampus; and its front pole stores the meanings of words and concepts.

Where is the temporal lobe located?
On the lower side of each cerebral hemisphere, below the lateral (Sylvian) sulcus that separates it from the frontal and parietal lobes, and in front of the occipital lobe. It sits roughly behind the temple.

Why is the temporal lobe important for memory?
Its medial structures - the hippocampus and the surrounding entorhinal, perirhinal and parahippocampal cortices - are needed to form new declarative memories. The patient H.M., who had these removed on both sides, lost the ability to remember new facts and events.

Who was patient H.M.?
Henry Molaison, who in 1953 had his medial temporal lobes removed on both sides to treat epilepsy. He was left unable to form new lasting memories while keeping his intelligence and older memories, and his case, studied by Brenda Milner, founded the modern science of memory.

What is the fusiform face area?
A patch of the fusiform gyrus, on the underside of the temporal lobe, that responds much more strongly to faces than to other objects. It is part of a mosaic of regions in the ventral temporal cortex specialised for categories such as faces, bodies, scenes and words.

What happens if the temporal lobe is damaged?
The deficit depends on where. Medial damage causes amnesia; damage to the ventral surface causes agnosia or an inability to recognise faces; damage to the posterior superior temporal cortex on the left impairs speech comprehension; and atrophy of the front pole causes the loss of word meaning in semantic dementia.

What is tonotopy?
The orderly mapping of sound frequency onto the auditory cortex, so that neighbouring patches of cortex respond to neighbouring frequencies. The map is logarithmic, so each doubling of frequency takes up a roughly equal stretch of cortex.

How is the temporal lobe different from the occipital lobe?
The occipital lobe is almost entirely visual. The temporal lobe is multifunctional: it handles hearing, the recognition of what is seen, the comprehension of language and the formation of memory, with these roles laid out across its different surfaces.

References