*3.3.2 Eleven human AFMs compose three cloverleaf clusters overlapping Heschl's gyrus*

With our new understanding of periodotopic representations overlapping the previously identified tonotopic gradients, *in vivo* fMRI measurements can now identify the 11 AFMs that compose the core and belt regions of human auditory cortex (**Figure 10**) [5, 10, 12, 23]. Running from STG to the circular sulcus (CiS) along HG are three distinct, concentrically organized, tonotopic representations. The primary circular tonotopic gradient is one dimension of the HG cloverleaf cluster, with a confluent low-tone representation located centrally and expanding smoothly to high-tone representations at the outer edge (**Figure 10B, C**) [5]. The HG cluster is divided along the orthogonal periodotopic reversals into two AFMs each of core, medial belt, and lateral belt: hA1, hR, hMM, hRM, hML, and hAL (**Figure 10D, E**). Positioned at the tip of HG, hA1 is the largest of these core and belt AFMs, with the posterior/lateral region representing low tones and the anterior/medial region representing high ones. HA1 is involved in the most basic of cortical auditory computations, which is reflected in its representations of broad ranges of tonotopy and periodotopy [2].

A reversal in the tonotopic gradient along the anteromedial edge of the HG cluster divides it from the CM/CL cluster just past the tip of HG (**Figure 10B, C**). A high-periodicity gradient reversal splits this tonotopic gradient into hCM, and hCL, two regions associated with early language and speech processing as well as audiovisual integration (**Figure 10D, E**) [164]. Finally, the reversal in the tonotopic gradient along the posteriolateral edge of the HG cluster separates it from the RT cluster positioned where HG meets STG (**Figure 10B, C**). Two reversals in

#### **Figure 10.**

*Auditory field maps and cloverleaf clusters in human cortex. (A) Anatomical views of Heschl's gyrus (HG), superior temporal gyrus (STG) and surrounding auditory cortex in an individual subject's left hemisphere (S2). (i) Inflated 3-D rendering of the cortical surface. Light gray denotes gyri; dark gray denotes sulci. The approximate region presented in the other panels is indicated by the dotted black line. Note that this subject has a double peak along HG. (ii) flattened cortical surface of the region indicated by the dotted black line in (i). AFM boundaries between maps along tonotopic reversals are indicated by solid black lines. These tonotopic reversals constitute the separation of cloverleaf clusters from one another. AFM boundaries along periodotopic reversals are indicated by dotted black lines. These periodotopic reversals compose the separation between maps within a cloverleaf cluster. Red text indicates AFM names. (B) Tonotopic gradients measured using narrowband noise stimuli with a phase-encoded fMRI paradigm (example single-subject data from [10]). Color overlay indicates the preferred frequency range for each voxel. CF: center frequency in Hz. For clarity, only voxels within the core and belt AFMs are shown. Solid and dotted black lines are as in (A). Coherence*  ≥*0.20. Inset scale bar designates 1 cm along the flattened cortical surfaces in (B, D). Inset legend indicates anatomical directions for (B-E). M: medial; L: lateral; A: anterior; P: posterior. (C) Diagram is based on individual-subject data measured by [10] in multiple phase-encoded fMRI experiments. Approximate positions of core AFMs (hA1, hR, hRT) are shown in white, and approximate positions of belt AFMs (hML, hAL, hRTL, hRTM, hRM, hMM, hCM, hCL) are shown in gray. Darker beige background indicates the plane of the lateral sulcus, while lighter beige overlay indicates gyri. Gyri are also marked with dashed black lines. HG: Heschl's gyrus. CG: circular gyrus; CiS: circular sulcus; a/p STG: anterior/posterior superior temporal gyrus. Diagram depicts the locations of tonotopic representations overlaid along the core and belt AFMs, with low (L) and high (H) tonotopic representations are marked in red and blue, respectively. Dotted black lines designate the boundaries between AFMs within three cloverleaf clusters: HG cluster with hA1; hCM/hCL cluster (partial cluster defined to date); hRTM/hRT/hRTL cluster (partial cluster defined to date). (D) Periodotopic representations measured using broadband noise stimuli with a phase-encoded fMRI paradigm. Data are from the same subject as shown for tonotopy in (B), with the color overlay now indicating the preferred period range for each voxel. AM rate: amplitude modulation rate in Hz. Other details are as in (B). (E) Diagram depicts periodotopic representations overlaid on the same example region of cortex as in (C). L and H now designate to the approximate locations of low (orange) or high (purple) periodotopic representations, respectively. Adapted from Barton et a.[10]. For a detailed review, see [5].*

**21**

**4. Conclusion**

*Attention and Working Memory in Human Auditory Cortex*

the periodotopic representations here divide the RT cluster into hRT, hRTM, and hRTL (**Figure 10D, E**). In macaque, these AFMs along STG are thought to subserve lower-level processing of auditory stimuli like temporally modulated environmental sounds [158, 159]. More research is needed to determine how what other AFMs form the CM/CL and RT clusters. Based on emerging data, it is likely that AFMs will also be a fundamental organization of auditory cortex adjacent to these cloverleaf

The characterization of AFMs and cloverleaf clusters will be crucial for the study of the structure and function of human auditory cortex, as these *in vivo* measurements allow for the systematic exploration of computations across a sensory system (for reviews, see [5, 17]). Such AFM organization provides a basic framework for the complex processing and analysis of input from the sensory receptors of the inner ear [5, 12, 17, 23]. The cloverleaf cluster organization of AFMs may also play a role in coordinating neural computations, with neurons within each cluster sharing computational resources such as common mechanisms to coordinate neural timing or short-term information storage [8, 12]. Similarly, vision studies suggest that functional specializations for perception are organized by cloverleaf clusters, as a particular cloverleaf cluster can be functionally differentiated from its neighbors by its pattern of BOLD responses, surface area, cortical magnification, processing specialization, and receptive field sizes [12, 16, 18, 19, 21, 165]. These distinctions indicate that CFMs within individual cloverleaf clusters are not only anatomically

The cluster organization is not necessarily thought to be driving common sensory functions, but rather reflects how multiple stages in a sensory processing pathway might arise during development across individuals and during evolution across species. It is likely that this cluster organization, like the topographic organization of CFMs, allows for efficient connectivity among neurons that represent neighboring aspects in sensory feature space [166–169]. Since the axons contained within one cubic millimeter of cortex can extend 3-4 km in length, efficient con-

The definitions of AFMs and the cloverleaf clusters they compose using phaseencoded fMRI will thus serve as reliable, independent localizers for investigations of attention and working memory in early auditory cortex across individuals. Measurements of individual AFMs along the cortical hierarchy will help reveal the distinct stages of top-down and bottom-up auditory processing. In addition, changes in AFMs can be tracked to study how auditory cortex changes under various attentional and working memory tasks and disorders (e.g., [145, 171–177]).

The human brain has sophisticated systems for perception, trace memory, attention, and working memory for audition and vision, and likely the other senses as well. These systems appear to be organized in a very similar manner for each sense, despite the inputs to each system and information content being quite different. Behavioral measures of the last several decades have led to the development of welldefined models of each system. These models form the basis for the investigation of their underlying architecture in the cortical structures of the human brain. EEG and PET have allowed for spatially coarse investigation of cortical activity, but with the advent of fMRI, it has become possible to make exceptionally detailed spatial

clusters, such as planum temporale (PT), planum polare (PP) and STG.

**3.4 Measuring attention and working memory in human AFMs**

*DOI: http://dx.doi.org/10.5772/intechopen.85537*

but also functionally related [15, 18, 20, 166].

nectivity is vital for sustainable energetics in cortex [170].

*Attention and Working Memory in Human Auditory Cortex DOI: http://dx.doi.org/10.5772/intechopen.85537*

*The Human Auditory System - Basic Features and Updates on Audiological Diagnosis and Therapy*

*Auditory field maps and cloverleaf clusters in human cortex. (A) Anatomical views of Heschl's gyrus (HG), superior temporal gyrus (STG) and surrounding auditory cortex in an individual subject's left hemisphere (S2). (i) Inflated 3-D rendering of the cortical surface. Light gray denotes gyri; dark gray denotes sulci. The approximate region presented in the other panels is indicated by the dotted black line. Note that this subject has a double peak along HG. (ii) flattened cortical surface of the region indicated by the dotted black line in (i). AFM boundaries between maps along tonotopic reversals are indicated by solid black lines. These tonotopic reversals constitute the separation of cloverleaf clusters from one another. AFM boundaries along periodotopic reversals are indicated by dotted black lines. These periodotopic reversals compose the separation between maps within a cloverleaf cluster. Red text indicates AFM names. (B) Tonotopic gradients measured using narrowband noise stimuli with a phase-encoded fMRI paradigm (example single-subject data from [10]). Color overlay indicates the preferred frequency range for each voxel. CF: center frequency in Hz. For clarity, only voxels within the core and belt AFMs are shown. Solid and dotted black lines are as in (A). Coherence*  ≥*0.20. Inset scale bar designates 1 cm along the flattened cortical surfaces in (B, D). Inset legend indicates anatomical directions for (B-E). M: medial; L: lateral; A: anterior; P: posterior. (C) Diagram is based on individual-subject data measured by [10] in multiple phase-encoded fMRI experiments. Approximate positions of core AFMs (hA1, hR, hRT) are shown in white, and approximate positions of belt AFMs (hML, hAL, hRTL, hRTM, hRM, hMM, hCM, hCL) are shown in gray. Darker beige background indicates the plane of the lateral sulcus, while lighter beige overlay indicates gyri. Gyri are also marked with dashed black lines. HG: Heschl's gyrus. CG: circular gyrus; CiS: circular sulcus; a/p STG: anterior/posterior superior temporal gyrus. Diagram depicts the locations of tonotopic representations overlaid along the core and belt AFMs, with low (L) and high (H) tonotopic representations are marked in red and blue, respectively. Dotted black lines designate the boundaries between AFMs within three cloverleaf clusters: HG cluster with hA1; hCM/hCL cluster (partial cluster defined to date); hRTM/hRT/hRTL cluster (partial cluster defined to date). (D) Periodotopic representations measured using broadband noise stimuli with a phase-encoded fMRI paradigm. Data are from the same subject as shown for tonotopy in (B), with the color overlay now indicating the preferred period range for each voxel. AM rate: amplitude modulation rate in Hz. Other details are as in (B). (E) Diagram depicts periodotopic representations overlaid on the same example region of cortex as in (C). L and H now designate to the approximate locations of low (orange) or high (purple) periodotopic representations, respectively. Adapted* 

**20**

*from Barton et a.[10]. For a detailed review, see [5].*

**Figure 10.**

the periodotopic representations here divide the RT cluster into hRT, hRTM, and hRTL (**Figure 10D, E**). In macaque, these AFMs along STG are thought to subserve lower-level processing of auditory stimuli like temporally modulated environmental sounds [158, 159]. More research is needed to determine how what other AFMs form the CM/CL and RT clusters. Based on emerging data, it is likely that AFMs will also be a fundamental organization of auditory cortex adjacent to these cloverleaf clusters, such as planum temporale (PT), planum polare (PP) and STG.

#### **3.4 Measuring attention and working memory in human AFMs**

The characterization of AFMs and cloverleaf clusters will be crucial for the study of the structure and function of human auditory cortex, as these *in vivo* measurements allow for the systematic exploration of computations across a sensory system (for reviews, see [5, 17]). Such AFM organization provides a basic framework for the complex processing and analysis of input from the sensory receptors of the inner ear [5, 12, 17, 23]. The cloverleaf cluster organization of AFMs may also play a role in coordinating neural computations, with neurons within each cluster sharing computational resources such as common mechanisms to coordinate neural timing or short-term information storage [8, 12]. Similarly, vision studies suggest that functional specializations for perception are organized by cloverleaf clusters, as a particular cloverleaf cluster can be functionally differentiated from its neighbors by its pattern of BOLD responses, surface area, cortical magnification, processing specialization, and receptive field sizes [12, 16, 18, 19, 21, 165]. These distinctions indicate that CFMs within individual cloverleaf clusters are not only anatomically but also functionally related [15, 18, 20, 166].

The cluster organization is not necessarily thought to be driving common sensory functions, but rather reflects how multiple stages in a sensory processing pathway might arise during development across individuals and during evolution across species. It is likely that this cluster organization, like the topographic organization of CFMs, allows for efficient connectivity among neurons that represent neighboring aspects in sensory feature space [166–169]. Since the axons contained within one cubic millimeter of cortex can extend 3-4 km in length, efficient connectivity is vital for sustainable energetics in cortex [170].

The definitions of AFMs and the cloverleaf clusters they compose using phaseencoded fMRI will thus serve as reliable, independent localizers for investigations of attention and working memory in early auditory cortex across individuals. Measurements of individual AFMs along the cortical hierarchy will help reveal the distinct stages of top-down and bottom-up auditory processing. In addition, changes in AFMs can be tracked to study how auditory cortex changes under various attentional and working memory tasks and disorders (e.g., [145, 171–177]).

## **4. Conclusion**

The human brain has sophisticated systems for perception, trace memory, attention, and working memory for audition and vision, and likely the other senses as well. These systems appear to be organized in a very similar manner for each sense, despite the inputs to each system and information content being quite different. Behavioral measures of the last several decades have led to the development of welldefined models of each system. These models form the basis for the investigation of their underlying architecture in the cortical structures of the human brain. EEG and PET have allowed for spatially coarse investigation of cortical activity, but with the advent of fMRI, it has become possible to make exceptionally detailed spatial

measurements. The methods of investigation must be carefully crafted to best elicit activity reflecting the desired aspects of each system; not only must the tasks be appropriate for fMRI, the stimuli and task must be closely matched not just to the system being studied, but to the inputs into that system as well.

For both audition and vision, the sensory processing in cortex happens in cloverleaf clusters of CFMs. This organizational pattern has clearly been demonstrated in the lower tiers of the processing hierarchy and very likely is organized as such throughout. Because the CFMs across the entire hierarchy (or at least, most) of one sense can be measured in just one session in the fMRI scanner, they make incredibly efficient localizers. CFMs are be measured in individual subjects, and serve as functional localizers that can be used to average more accurately across subjects than anatomical localizers. As such, due to the pervasive and fundamental role CFMs play in sensory systems, they are also excellent candidates for measuring the effects of attention and working memory in cortex. To best accomplish this feat, it is proposed that stimuli that are similar to those used to measure CFMs are excellent candidates for use in traditional tasks used to define attentional and working-memory models.
